Karen defeated; new disturbance a threat to the Gulf of Mexico

Wind shear put an end to Tropical Storm Karen yesterday. Karen's remains continue to generate a large area of disturbed weather a few hundred miles east of the northern Lesser Antilles Islands (Figure 1). This morning's QuikSCAT pass showed some rotation still exists near the surface, with winds up to 30 mph. Wind shear has fallen to 20 knots over Karen's remains, and some of the models forecast the shear will fall below 15 knots by Tuesday, which may allow redevelopment of the storm. Both the UKMET and NOGAPS model revive Karen later this week, and show it moving very slowly to the west towards the U.S., but well offshore. The GFS model keeps the shear high, and does not redevelop Karen.

Figure 1. This morning's visible satellite image.

Bahamas tropical disturbanceA potential significant threat area has developed today off the U.S. East Coast along an old frontal boundary. Several areas of heavy thunderstorm activity have started firing up along this old front. Wind shear is about 20 knots over the region today, so only slow development will occur. By Tuesday, the shear is forecast to drop below 15 knots, and most of the computer models are forecasting that a tropical depression will form near Florida or Cuba. This storm is forecast to move westward across the Gulf of Mexico, pushed by a strong ridge of high pressure expected to build in. An upper-level anticyclone aloft is expected to develop as well, providing an environment favorable for intensification. The UKMET model is forecasting a strong tropical storm or weak hurricane by Friday for the Texas/Mexico border region. The other models are not so aggressive, but all see the possibility of a tropical storm impacting the western Gulf of Mexico in Louisiana, Texas, or Mexico sometime Thursday through Saturday. The Hurricane Hunters are on call to fly into this system Tuesday afternoon, if necessary. Texas is at highest risk from this potential storm.

MelissaTropical Depression Melissa is not long for this world, thanks to wind shear of 15-20 knots that is expected to increase further as Melissa heads northwest. Melissa is not a threat to any land areas, and will not be around three days from now.

Karen struggling; Lorenzo sets a new intensification record

Tropical Depression Karen has weakened to a tropical depression, thanks to ferociously high wind shear levels exceeding 50 knots. Satellite loops show Karen's exposed low-level center of circulation, visible as a swirl of low clouds. Karen continues to generate new heavy thunderstorm activity, but these cells immediately get pushed several hundred miles east of the center by strong upper level winds from the west. This morning's QuikSCAT pass missed Karen.

Wind shear is expected to remain above 30 knots over Karen through Sunday, then decline to 15 knots by Sunday night. Although the storm has shown an unusual ability to maintain a strong wind pattern in the face of high wind shear, it is uncertain if Karen can survive past today. The GFS and ECMWF models predict the shear will destroy Karen. I put Karen's survival chances at about 25%.

If Karen survives, an upper level environment favorable for strengthening is expected to set up 3-5 days from now, and Karen would probably become a hurricane. Steering currents may become weak during that period, and Karen may move very slowly. By the middle of next week, I expect a ridge of high pressure will build over the eastern U.S. and western Atlantic, forcing Karen westward towards the U.S. The path Karen might take late next week is highly uncertain. Most of the models continue to predict the formation of a new tropical or subtropical cyclone somewhere between the coast of North Carolina and the Western Caribbean on Monday. Some of the models take this new storm northeastward out to sea, which would pull Karen northward in its wake. However, most of the models predict that the new storm will take a more westerly path into Georgia or Florida, or possibly the Gulf of Mexico. A storm-storm interaction between Karen and the new storm might ensue, an event the models are poor at handling.

Figure 1. This morning's visible satellite image with wind shear contours overlaid show a very unhealthy tropical depression. Wind shear of 50 knots is over the low-level circulation center of Karen, thanks to strong upper-level winds from the west (denoted by the big white arrow at the plot's bottom). These high winds have pushed all of Karen's heavy thunderstorm activity several hundred miles downwind of the center. Image credit: University of Wisconsin CIMSS.

Lorenzo sets another rapid intensification record for 2007Hurricane Lorenzo hit Mexico's Veracruz coast near Tuxpan early Friday morning. The storm's heavy rains have triggered mudslides blamed for at least four deaths. Lorenzo has tied the Atlantic record for fastest intensification from a tropical depression to a Category 1 hurricane--twelve hours. Hurricane Blanche of 1969 was the only other storm on record that intensified from a tropical depression to a Category 1 hurricane in just 12 hours. Hurricane Ethel of 1960 may have done so faster, though. Ethel strengthened from a 45-mph tropical storm to a 85 mph Category 1 hurricane in just 6 hours. We don't know when Ethel started as a tropical depression, since this was before the satellite era.

Reliable record keeping of intensification rates of Atlantic hurricanes began in 1970, when regular satellite coverage became available. Since 1970, Hurricane Humberto of 2007 holds the record for fastest intensification from first advisory issued to hurricane strength--18 hours. (Actually, Humberto did the feat in 14 1/4 hours, but this will get rounded off to 18 hours in the final data base, which stores points every six hours). There have been six storms that accomplished the feat in 24 hours.

Since 1970, Hurricane Felix of 2007 holds the record for fastest intensification from the first advisory to a Category 5 hurricane. It took Felix just 54 hours to accomplish the feat. Hurricane Camille of 1969 also took 54 hours to do so, but the first advisory put Camille as a 60 mph tropical storm. It is likely that Camille would have been classified as a tropical depression earlier had reliable satellite imagery been available.

Is it a statistical fluke that we've had three record-speed intensifying hurricanes this year? It could be. Our reliable data records only go back to 1970, and there may have been periods in the past with similar events. No scientist has published a paper linking rapid hurricane intensification rates with global warming. However, three record-speed intensifying hurricanes in one season certainly raises questions, and is very odd.

MelissaTropical Storm Melissa formed this morning, far out in the eastern Atlantic. The storm is under about 10-20 knots of wind shear. Melissa will probably not affect land, since it is starting out too far north and will gain additional latitude in the coming days. The storm's expected track will take it northwest towards a region of high wind shear early next week, which should destroy the storm.

Hurricane Lorenzo blew ashore at about midnight local time on the Mexican coast as a Category 1 hurricane with 75 mph winds. Satellite imagery shows that this very small storm is already starting to dissipate, and Lorenzo's effects will be confined to a small area near the coast where the storm made landfall. A pass from NASA's TRMM satellite (Figure 1) showed rainfall amounts of up to one inch per hour from Lorenzo. Lorenzo could dump 5-10 inches of rain over a narrow region extending 50 miles inland from the landfall location. Widespread major flooding is not expected, but some serious local flooding will probably occur.

Figure 1. Rainfall rates estimated by NASA's TRMM satellite as Lorenzo made landfall at 11:26 pm EDT last night. Rainfall amounts as high as one inch per hour (red colors) were estimated in the eyewall and in the spiral band to the west of Lorenzo.

It's not unusual for tropical storms to intensify suddenly where Lorenzo did, in the extreme southwestern Gulf of Mexico's Bay of Campeche. According to a paper presented by NASA's Scott Braun at a hurricane conference in 2006, this may be due to the mountains along shore diverting the low-level winds and helping intensify a tropical storm's vortex. In the case of a model simulation done of 2005's Tropical Storm Gert the authors write: "The simulation shows an easterly wave and surface trough moving westward over the southern Gulf of Mexico with somewhat disorganized convection occurring in its vicinity. Low-level easterly flow ahead of the trough impinges on the eastern side of the Sierra Madre mountains and leads to flow blocking. Because of the particular shape of the topography, this blocked flow causes northwesterly to westerly flow to occur in the southern Gulf that eventually pinches off the trough to form a closed cyclonic circulation. This topographically forced flow also helps to organize some of the convection in a linear band south of the vortex center. Once the closed cyclonic circulation is formed, convection increases and gradually intensifies the cyclone to tropical storm strength by the time of landfall.

Tropical Storm KarenTropical Storm Karen has weakened to a minimal tropical storm, thanks to continued high levels of wind shear. Strong westerly winds aloft are creating about 20-25 knots of wind shear over Karen. This morning's QuikSCAT pass showed a very elongated center of circulation, with winds of minimal tropical storm force to the east of the center. Satellite loops show Karen's exposed low-level center of circulation, now visible as a swirl of low clouds. Most of the heavy thunderstorm activity that was was on the storm's northeast side has died out this morning, and Karen may get downgraded to a tropical depression today. A small area of heavy thunderstorm activity has developed near the center on the east side, but Karen is looking very unhealthy now.

Karen has taken an unexpected jog to the north this morning, which puts the storm closer to a region of higher wind shear. A motion to the west-northwest is expected to resume later today. With Karen's more northerly position, the odds of the storm being torn apart have increased. The GFS model kills Karen by Sunday, and the SHIPS model (which is based on the GFS) is forecasting that 35-40 knots of wind shear will affect the storm tonight through Saturday night. The other models do not kill Karen, but none of them forecasted the northward jog that will bring Karen into higher wind shear sooner. There is about a 50% chance Karen will be gone by Sunday, destroyed by wind shear.

If Karen survives, an ominous possibility arises--a ridge of high pressure is expected to build over the eastern U.S. and western Atlantic, forcing Karen westward towards the U.S. An anticyclone with low wind shear might build over the storm, creating an environment favorable for intensification of Karen into a hurricane. The path Karen might take late next week is highly uncertain. The high pressure ridge expected to build to the north of Karen will be quite strong, which might force Karen westward into Florida, or even west-south-westward through the Bahamas and into Cuba. There will be several small "short wave" troughs moving through the ridge that may be able to turn Karen more northwesterly towards New England, though. Still another uncertainty is the possibility of a new tropical, subtropical, or extratropical cyclone forming off the coast of North Carolina on Sunday or Monday, something now predicted by all of the computer models. Some of the models take this new storm north-eastward out to sea, which would create a weakness in the ridge that would pull Karen northward. However, most of the models predict that the new storm will move west or west-southwest over Georgia or Florida. A storm-storm interaction between Karen and the new storm might ensue, an event the models are poor at handling.

In short, this is a very complicated situation, and we'll just have to wait and see how it unfolds.

Tropical Depression 14Tropical Depression 14 formed about 50 miles southwest of the Cape Verdes Islands this morning. The storm is under about 10-20 knots of wind shear. This shear should keep any development slow today. This morning's high-resolution QuikSCAT pass showed that the storm down not have a well-organized surface circulation, but rather a crescent-shaped line of converging winds arcing along a 400-mile long line. A few 50-knot wind vectors appeared in squalls on the storm's east side. This system will likely never affect land, since it is starting out too far north and will gain additional latitude in the coming days. The storm is headed northwest towards a region of high wind shear, and may not survive long.

Karen, TD 13, Florida disturbance 98L, and African disturbance

Tropical Storm Karen is being ripped apart by 20-40 knots of wind shear, thanks to strong southwesterly winds aloft (Figure 1). Satellite loops show that these strong winds have exposed the center of circulation, now visible as a swirl of low clouds, and pushed the remaining heavy thunderstorm activity to the storm's northeast side.

Karen was probably a hurricane yesterday morning, since a NOAA hurricane hunter aircraft that arrived at the storm during the afternoon found winds near hurricane force. These winds were much stronger than the storm's satellite presentation suggested. This flight occurred after Karen had already peaked in intensity, so it is likely Karen was a hurricane for a few hours. The storm may be upgraded to a hurricane in post-storm analysis. It looks right now that Karen does not have much chance to regain hurricane status over the next five days, and may not survive at all. Wind shear over the next five days is predicted to range between 20-30 knots, and there is at least a 30% chance Karen will be destroyed by this. None of the computer models forecast total destruction, though.

Figure 1. This morning's visible satellite image, with yellow contour lines showing the amount of wind shear (in knots) superimposed. Strong upper winds winds blowing from the southwest (shown by the large white arrow) were creating 20-40 knots of wind shear over Karen. These strong winds pushed Karen's heavy thunderstorm activity to the downwind (northeast) side of Karen, exposing the low-level center of circulation to view. Image credit: University of Wisconsin CIMSS.

A severely weakened Karen will not turn to the north as much as the models have been predicting, since the storm will not extend as high in the atmosphere. Shallow storms respond to the wind field closer to the surface. These surface winds are blowing more east-to-west, and Karen is expected to follow a more westerly track to a point a few hundred miles north of the Lesser Antilles Islands five days from now. Steering currents will weaken then, and Karen will move slowly for a few days. It appears likely now that a ridge of high pressure will then build in and force Karen (or its remnants) to the west towards the U.S. East Coast late next week.

Tropical Depression 13Tropical Depression 13 remains virtually unchanged from a day ago. Satellite imagery shows a small, compact storm that is affecting a very limited portion of the Mexican coast. Wind shear is a low 5 knots, and TD 13 could still strengthen to a 50-55 mph tropical storm before its expected landfall in Mexico on Saturday, between Poza Rica and Tampico. TD 13 will not affect Texas, due to the storm's small size. The next Hurricane Hunter mission is scheduled to arrive at 2 pm EDT today.

Tropical disturbance 98L near the east coast of FloridaA surface low pressure area (98L) moved over South Florida last night, and now appears to be reforming about 100 miles southeast of Cape Canaveral. The system is bringing heavy rain to the waters offshore Florida and the northern Bahama Islands, as seen on long range radar out of Miami. However, there are no organized spiral bands, and winds measured throughout Florida this morning have been 8 mph or less. The system is under about 10-15 knots of wind shear, which may allow some development today. This disturbance has brought rains of up to four inches to portions of South Florida and the western Bahamas, as seen on Miami radar. The center is forming far enough off the coast that 98L will probably not be a big rainmaker for Florida.

The disturbance is lifting northeastward in response to a strong trough of low pressure swinging off the U.S. East Coast. The system has the potential to organize into a tropical depression today or tomorrow, and 98L will pass several hundred miles east of Cape Hatteras, North Carolina Saturday. The system is expected to accelerate to the northeast and could bring heavy rain and high wind gusts to the Canadian Maritime provinces early next week. It is unlikely 98L will have time to become any stronger than a 45 mph tropical storm, and any effects on the U.S. will be minimal.

Coast of Africa waveA tropical wave about 150 miles south of the Cape Verdes Islands is under about 10-20 knots of wind shear, and has some potential for development over the next few days. This morning's QuikSCAT pass showed an elongated circulation, with top winds of 35 mph to the storm's northeast side.

Tropical update

Tropical Storm Karen is now suffering the effects of 15-20 knots of winds shear, and has stopped intensifying. The latest satellite loops show a large and expanding cloud pattern with good outflow to the north, but no eye. The wind shear affecting Karen is expected to stay at 15-20 knots the next four days, and Karen will probably not change much in strength during that period. The long range outlook for Karen remains unchanged. The models are all fairly unified in taking Karen to a latitude north of the Lesser Antilles Islands. However, the trough of low pressure expected to steer Karen to the northwest is unlikely to be strong enough to recurve Karen out to sea. A new ridge of high pressure may build in, forcing Karen westwards towards the U.S., just north of the Lesser Antilles Islands. This scenario, favored by the UKMET model, would put Karen in a position to threaten the U.S. East Coast late next week. The GFS model does not build in such a strong ridge, and instead forecasts that Karen will stall for 2-4 days a few hundred miles north of the Lesser Antilles Islands, and eventually recurve out to sea when the next strong trough of low pressure comes along. This scenario would put Bermuda at risk from Karen. It is too early to speculate which of these scenarios is more likely, and how much of a risk Karen may present to Bermuda or the U.S. East Coast.

Tropical Depression 13Tropical Depression 13 remains nearly stationary in the far southwestern Gulf of Mexico. Satellite imagery show a well-organized system that should be a tropical storm, but this is not supported by Hurricane Hunter reports. The top winds found this afternoon at the surface by the airplane were less than 30 mph. Wind shear is about 10 knots over the disturbance, and is expected to remain 10 knots or below for the next three days. This should allow Tropical Depression 13 to develop into a strong tropical storm--possibly a Category 1 hurricane--before it makes landfall along the Mexican coast 2-3 days from now. Steering currents are weak in the southwest Gulf of Mexico, and the storm will move slowly and erratically. This storm is primarily a threat to Mexico, due to a very strong ridge of high pressure that will prevent the storm from turning northwards.

Tropical wave 97L grows disorganizedA tropical wave (97L) has brought heavy rains of up to 2-4" to Puerto Rico today. Satellite imagery and Puerto Rico long range radar show that this wave has degenerated. High wind shear of 20-30 knots has seriously disrupted the wave, and this will be my last mention of it.

Tropical disturbance 98L near South Florida and the BahamasA surface low pressure area (98L) formed near Key West this morning, and has moved northeast to a position off the SW coast of Florida south of Naples. The rotation of the low is clearly evident on long range radar out of Miami. However, there are no organized spiral bands, and winds measured throughout South Florida this afternoon have been 12 mph or less. The system is under about 10-20 knots of wind shear, which should keep any development slow today. This disturbance has brought rains of up to four inches to portions of the Florida Keys and the western Bahamas as seen on Miami radar.

The disturbance is lifting northeastward in response to a strong trough of low pressure swinging off the U.S. East Coast. On Thursday, when 98L will be over the warm waters of the Gulf Stream approaching North Carolina, the system has the potential to organize into a tropical depression, as wind shear is expected to fall to 10 knots. The GFDL develops 98L very rapidly, and predicts it will approach Category 1 hurricane status as it passes about 200 miles east of Cape Hatteras on Friday night. This is too aggressive a forecast, and I prefer the HWRF forecast, which forecasts top winds of 35-40 mph by Friday night. A very strong band of wind shear associated with the jet stream will affecting 98L by Friday night, and could rapidly destroy the storm. 98L or its remnants may then move rapidly northeastward past Cape Cod, Massachusetts, or may stall off the coast of North Carolina and wander erratically for several days. The models are split on this matter.

Coast of Africa waveA tropical wave a few hundred miles south-southeast of the Cape Verdes Islands is under about 10 knots of wind shear, and has some potential for development over the next few days.

Tropical Storm Karen is midway between Africa and the Lesser Antilles Islands. Karen is steadily intensifying, as seen in the latest satellite loops. Low level spirals bands continue to organize, and upper-level outflow is becoming established to the north and south. Wind shear of 10-20 knots is keeping intensification slow,and Karen may not be able to attain hurricane strength this week. Hostile wind shear in association with a trough of low pressure is expected to affect Karen over the next few days, and this trough will also turn the storm to the north or northwest. The models are all fairly unified in taking Karen to a latitude north of the Lesser Antilles Islands. However, the trough of low pressure expected to steer Karen to the northwest is unlikely to be strong enough to recurve Karen out to sea. A new ridge of high pressure may build in, forcing Karen westwards towards the U.S., just north of the Lesser Antilles Islands. This scenario, favored by the UKMET model, would put Karen in a position to threaten the U.S. East Coast late next week. The GFS model does not build in such a strong ridge, and instead forecasts that Karen will stall for 2-4 days a few hundred miles north of the Lesser Antilles Islands, and eventually recurve out to sea when the next strong trough of low pressure comes along. This scenario would put Bermuda at risk from Karen. It is too early to speculate which of these scenarios is more likely, and how much of a risk Karen may present to Bermuda or the U.S. East Coast.

Tropical Depression 13Tropical Depression 13 formed yesterday evening in the far southwestern Gulf of Mexico. Satellite imagery and this morning's QuikSCAT pass show a well-organized surface circulation. Most of the winds seen by QuikSCAT were contaminated by rain, so it is difficult to say what the winds are. Wind shear is about 10 knots over the disturbance, and is expected to fall below 5 knots for the next three days. This should allow Tropical Depression 13 to develop into a strong tropical storm--possibly a Category 1 hurricane--before it makes landfall along the Mexican coast 2-3 days from now. Steering currents are weak in the southwest Gulf of Mexico, and the storm will move slowly and erratically. This storm is primarily a threat to Mexico, due to a very strong ridge of high pressure that will prevent the storm from turning northwards. The next Hurricane Hunter flight is scheduled for 2 pm EDT this afternoon.

Tropical wave 97L bringing heavy rain to Puerto RicoA tropical wave (97L) has brought heavy rains of up to 2-4" to Puerto Rico today. Satellite imagery and Puerto Rico long range radar show heavy but disorganized thunderstorm activity surrounding Puerto Rico and spreading to the Dominican Republic. These heavy rains will spread over northern Haiti Thursday as the wave tracks west-northwest at 10-15 mph. The wave no longer has a closed circulation, and is under 20-30 knots of wind shear. This shear is expected to remain 20-30 knots through Thursday afternoon, preventing any development. When the wave arrives in the eastern Bahamas Thursday night, wind shear is expected to drop to 10 knots, and stay 10-20 knots through Saturday. This may allow some slow development. The UKMET model is forecasting that 97L will become a tropical depression near South Florida on Saturday or Sunday, but none of the other models go along with this prediction.

Figure 2. Radar estimated rainfall from 97L over Puerto Rico.

Disturbed weather over Florida and the BahamasAn area of disturbed weather has formed over South Florida and the western Bahama Islands, in association with an upper-level trough of low pressure. NHC has labeled this system "98L" this morning. The region is under about 15-20 knots of wind shear, which should keep any development slow today. This disturbance has brought rains of up to four inches to portions of the Florida Keys and the western Bahamas as seen on Miami radar. The thunderstorm activity associated with 98L is currently disorganized, as seen on long range radar out of Miami. The disturbance is expected to lift northeastward in response to a strong trough of low pressure swinging off the U.S. East Coast. On Thursday, when 98L will be over the warm waters of the Gulf Stream approaching North Carolina, the system has the potential to organize into a tropical depression, as wind shear is expected to fall to 10-20 knots. By Friday, the GFS model predicts 98L should be moving rapidly northeastward past Cape Cod, Massachusetts, and could bring heavy rain and high winds to Nantucket, Martha's Vinyard, and eastern Massachusetts. This would not give 98L much time to organize, and at worst 98L should become a weak tropical storm with 45 mph winds. However, the Canadian model predicts that the trough of low pressure pulling 98L northeast will not be strong enough to finish the job, and the storm will stall off the North Carolina coast. I'll have more on this possibility later.

More action off the coast of AfricaAn area of disturbed weather has moved off the coast of Africa, and is located a few hundred miles south of the Cape Verdes Islands. This tropical wave is under about 10 knots of wind shear, and has some potential for development over the next few days. Most of the computer models forecast that a tropical depression will form off the coast of Africa in the next 2-5 days.

A very busy day in the tropics; wind shear tutorial posted

Satellite imagery and this morning's QuikSCAT pass show a vigorous surface circulation that continues to spin in the Gulf of Mexico, about 280 miles south-southeast of Brownsville, Texas. This disturbance (94L) has one clump of heavy thunderstorms on the northern side of the circulation, and QuikSCAT saw winds up to 25 mph at 8:14 am this morning. Wind shear has fallen to about 10 knots over the disturbance, and is expected to remain 10 knots or below for the next three days. This should allow 94L to develop into a tropical depression later today or on Wednesday. Steering currents are weak in the southwest Gulf of Mexico, and 94L will move slowly and erratically. This storm is primarily a threat to Mexico, due to a very strong ridge of high pressure expected to remain in place over the Gulf of Mexico the rest of the week. Several of the models foresee an eventual landfall to the west or south between Tampico and Campeche late this week. A Hurricane Hunter flight is scheduled to investigate 94L this afternoon.

Lesser Antilles disturbance 97L needs to be watchedA tropical wave (97L) was near 15N 62W, or just west of Dominica and Martinique in the Lesser Antilles Islands at 8 am EDT this morning. This wave had a closed circulation yesterday, but this circulation degenerated and was no longer fully closed on this morning's QuikSCAT pass. Top winds from QuikSCAT were about 30 mph just to the northeast of the northernmost islands in the chain. None of the islands had winds exceeding 15 mph between 8 am and 9 am EDT this morning. Satellite imagery shows heavy thunderstorm activity mostly died out last night, but is now making a comeback, with some major cells blowing up over Guadaloupe. A surface circulation also appears to be trying to form under these new cells. The wave is under about 10-20 knots of wind shear. The shear is forecast to remain near 20 knots through Thursday, which may allow some slow development. The 06Z (2 am EDT) run of the GFDL model did develop 97L into a weak tropical storm. The forecast had 97L moving over Puerto Rico on Wednesday, the Dominican Republic on Thursday, and eastern Cuba by Saturday. The 06Z HWRF model did not develop 97L. If 97L does develop, and survives passage over Hispaniola and Cuba, and survives some possible encounters with areas of high wind shear, it could arrive at the Florida coast on Sunday as a tropical storm. That's a lot of "ifs", and there is only about a 5% chance of this happening. The Hurricane Hunter mission for this afternoon into 97L was canceled, and no new missions are scheduled.

Figure 1. Microwave image from 4:39 am EDT today showing low-level spiral banding coming together in Karen. This image is bit different from the usual 85 GHz images I show, which emphasize where it is raining through the total depth of the storm. The image above is taken at 37 GHz, where one sees more of the surface structure of the storm. Image credit: Navy/NRL.

Tropical Storm Karen formsTropical Storm Karen formed this morning, midway between Africa and the Lesser Antilles Islands. This large storm is still disorganized, as seen in the latest satellite loops. Low level spirals bands are slowly organizing, and with wind shear 10 knots or less, Karen should be able to steadily strengthen to a hurricane, as predicted by the SHIPS and GFDL intensity models. The HWRF model keeps Karen just below hurricane strength. Later this week, Karen may encounter a region of hostile wind shear in association with a trough of low pressure that will also turn the storm to the north or northwest. This may slow or reverse Karen's intensification.

It currently appears that Karen will not affect any land areas. It is unusual for a hurricane to turn west and hit the U.S. after going as far north as the official NHC forecast has the storm five days from now. The GFS model has consistently shown that Karen will recurve out to sea east of Bermuda next week. However, the UKMET model is showing a more southerly track just north of the Lesser Antilles islands six days from now. Should Karen follow this track, the storm may miss the trough the GFS is predicting will recurve it. In this case, Karen could be a long-range threat to Bermuda or the U.S. East Coast.

Disturbed weather over Florida and the BahamasAn area of disturbed weather has formed over South Florida and the western Bahama Islands, in association with an upper-level trough of low pressure. The region is under about 20 knots of wind shear, which should prevent any development. This disturbance is bringing heavy rain to portions of South Florida, the Bahamas, and Cuba, as seen on long range radar out of Key West.

Hurricane Center Directorship openThe position of director of the National Hurricane Center is officially open, according to a story published today in the Miami Herald. The current acting director, veteran hurricane forecaster Ed Rappaport, will decline to apply. "The timing of this position is not right, both personally and professionally, so I have chosen not to apply," said Rappaport, 49. Leading candidates for the job include the current interim deputy director, Bill Read, and hurricane specialist Rick Knabb.

Three tropical depressions may form by Wedneday in the Atlantic

Satellite imagery this morning shows that a new surface circulation has developed in the Gulf of Mexico near 23N 93W, about 350 miles southeast of Brownsville, Texas. This new circulation has been labeled 94L by NHC this morning. Heavy thunderstorm activity has begun forming near the center of circulation, and this system has the potential to become a tropical depression by Tuesday. Wind shear is about 15-20 knots over the disturbance, and is expected to remain 15 knots or below for the next three days. The disturbance is headed west-northwest at less than 10 mph, and should bring heavy rains to northern Mexico--and possibly southern Texas--by Wednesday. However, most of the computer models show that 94L may stall before it reaches the coast, then loop erratically in the Gulf for several days. A Hurricane Hunter flight is scheduled to investigate 94L Tuesday afternoon.

Figure 1. Today's lineup of tropical systems to watch.

Lesser Antilles disturbance 97L getting more organizedA tropical wave (97L) is just north of Barbados in the Lesser Antilles Islands. This wave has developed a closed circulation centered over the southern Lesser Antilles Islands, as seen on the 9:10 am EDT pass from the ASCAT satellite. This is confirmed by wind observations from Barbados this afternoon, where the winds have turned to westerly and increased to 20 mph. The latest Satellite imagery shows a marked increase in heavy thunderstorm activity in the past few hours, and a tropical depression could form by Tuesday. The wave is under about 10 knots of wind shear. The shear is forecast to slowly rise to 20 knots by Thursday, which may slow intensification. The future evolution of the storm depends on how close it comes to the mountainous island of Hispaniola. Most of the models predict 97L will pass over the island on Wednesday or Thursday, which would greatly disrupt the storm. None of the models predict 97L will grow stronger than a 55 mph tropical storm for the next four days. There is a band of very high wind shear predicted to lie just north of Hispaniola all of this week, and 97L could well encounter this band of high wind shear Thursday, which would weaken the storm. In any case, this system represents a threat to the Dominican Republic, Haiti, Puerto Rico, and the northern Lesser Antilles Islands, and residents may experience tropical storm conditions as early as Wednesday afternoon. The Hurricane Hunters are on call to investigate 97L Tuesday afternoon, if necessary. This afternoon's flight was canceled.

Atlantic disturbance 96L midway between Africa and the Lesser Antilles nearing tropical depression strengthA tropical wave (96L) near 9N 33W, midway between Africa and the Lesser Antilles Islands, is moving west-northwest at 10-15 mph. This wave has gotten much more organized this afternoon, as seen in the latest satellite loops. The circulation associated with the wave is unusually large. The storm has been slow to organize, since it is so large and so far south. At the storm's current latitude--9 degrees north of the Equator--it cannot leverage the earth's spin very much to help spin up the huge circulation it has. However, the storm will probably be a tropical depression by Tuesday morning. Low-level spiral bands have already formed, as seen in recent microwave satellite images (Figure 2). This morning's 7:29 am EDT ASCAT pass showed a better defined circular wind pattern with top winds of 25-30 mph. The wave is under about 10 knots of wind shear. The shear forecast is problematic, with some of the models expecting high wind shear later this week, and others keeping the wind shear low. I expect 96L will become at least a weak tropical storm by Wednesday, then we'll have to see how the upper level winds evolve. The storm is expected to gradually work its way north as it crosses the Atlantic, and appears likely to pass north of the Lesser Antilles Islands next week. The last few runs of the GFS model show 96L eventually recurving out to sea next week.

Tropical Storm JerryTropical Depression Jerry is only worth mentioning since it increases our storm totals for the year to ten. Jerry will be gone tonight, absorbed by an extratropical low pressure system.

Hurricane Rita anniversaryToday marks the second anniversary of Hurricane Rita's landfall in Southwest Louisiana. Wunderblogger Mike Theiss has written a blog and posted his usual amazing photos documenting his experience with Hurricane Rita. One of the more remarkable features of the account is his encounter with hundreds of exhausted birds in the eye of Rita. The unfortunate birds got trapped in the eye for days, unable to escape.

Tropical depressions may form in the Gulf of Mexico and the eastern Caribbean in the next day

A disturbance in the Gulf of Mexico moved ashore into Louisiana early this morning and is no longer being tracked by NHC as "Invest 94". Long range radar out of New Orleans shows a steady stream of moisture associated with the disturbance continues to flow northwards into Louisiana today. Satellite imagery this morning shows that a new surface circulation has developed in the Gulf of Mexico near 23N 93W, about 350 miles southeast of Brownsville, Texas. This new circulation has been labeled 94L by NHC this morning. Heavy thunderstorm activity has begun forming near the center of circulation, and this system has the potential to become a tropical depression tonight or Tuesday. Wind shear has dropped to about 15 knots over the disturbance, and is expected to remain 15 knots or below for the next three days. The disturbance is headed west-northwest at less than 10 mph, and should bring heavy rains to northern Mexico--and possibly southern Texas--by Wednesday.

Figure 1. Today's lineup of tropical systems to watch.

Lesser Antilles disturbance 97LA tropical wave (97L) is very near Barbados in the Lesser Antilles Islands. This wave has not gotten any better organized during the past 24 hours, as seen in the latest Satellite imagery. The wave is under about 10 knots of wind shear. The shear is forecast to slowly rise to 20 knots by Thursday. There is some favorable anticyclonic outflow at high levels, and 97L has a good chance of developing into a tropical depression by Wednesday. At that point, the future evolution of the storm depends on how close it comes to the mountainous island of Hispaniola. Most of the models predict 97L will pass over the island, which would greatly disrupt the storm. The GFDL model predicts 97L will survive the disruption, and re-intensify as it continues to move northwest into the easternmost Bahama Islands on Saturday. However, there is a band of very high wind shear predicted to lie just north of the Bahamas all of this week, and 97L could well encounter this band of high wind shear Thursday, which should weaken the storm. In any case, this system represents a threat to the Dominican Republic, Haiti, and Puerto Rico on Wednesday and Thursday, and residents may experience tropical storm conditions as early as Wednesday morning. The Hurricane Hunters are on call to investigate 97L Tuesday afternoon, if necessary. This afternoon's flight was canceled.

Figure 1. Microwave image from 3:28 am EDT today showing low-level spiral bands starting to form on the west side of 96L. Image credit: Navy/NRL.

Atlantic disturbance 96L midway between Africa and the Lesser AntillesA tropical wave (96L) near 9N 32W, midway between Africa and the Lesser Antilles Islands, is moving west-northwest at 10-15 mph. This wave has gotten much more organized late this morning, as seen in the latest satellite loops. The circulation associated with the wave is unusually large, and one can see a large area of inflowing low-level cumulus clouds spiraling into the center of 96L. The storm has been slow to organize, since it is so far south. At the storm's current latitude--9 degrees north of the Equator--it cannot leverage the earth's spin very much to help spin up the huge circulation it has. However, the storm will probably be a tropical depression by Tuesday morning. A few low-level spiral bands have already formed, as seen in recent microwave satellite images (Figure 1). Last night's QuikSCAT pass showed an elongated circulation, with winds generally in the 15-20 mph range. The wave is under about 10 knots of wind shear. The shear forecast has changed significantly since yesterday, and high levels of wind shear exceeding 20 knots are now expected to impact 96L beginning Tuesday, rising to 30 knots on Thursday. This is the type of wind shear Tropical Storm Ingrid encountered earlier this month, and the shear eventually destroyed the storm. Both the GFDL and HWRF models develop 96L into a tropical storm, but keep it a minimal tropical storm through the end of their 5-day forecast period. The storm is expected to gradually work its way north as it crosses the Atlantic, and appears likely to pass north of the Lesser Antilles Islands next week. The last few runs of the GFS model show 96L eventually recurving out to sea next week.

Tropical Storm JerryTropical Storm Jerry is only worth mentioning since it increases our storm totals for the year to ten. Jerry will be gone tomorrow, absorbed by an extratropical low pressure system.

Hurricane Rita anniversaryToday marks the second anniversary of Hurricane Rita's landfall in Southwest Louisiana. Wunderblogger Mike Theiss has written a blog and posted his usual amazing photos documenting his experience with Hurricane Rita. One of the more remarkable features of the account is his encounter with hundreds of exhausted birds in the eye of Rita. The unfortunate birds got trapped in the eye for days, unable to escape.

Active tropics today

A disturbance ("94L") in the Gulf of Mexico has not gotten better organized today and has just about run out of time. Long range radar out of New Orleans shows the advance rain showers from 94L are already at the coast, and there are no signs of spiral banding, rotation, or organization in either the radar imagery or satellite loops of 94L. Wind shear has increased to 10-20 knots over the Gulf of Mexico, and it now appears unlikely 94L will be able to develop into a tropical depression.

Lesser Antilles disturbance 97LOf greater concern is a tropical wave (97L) about 300 miles east of Barbados in the Lesser Antilles Islands. This wave has not gotten any better organized during the past 12 hours, as seen in the latest Satellite imagery. The wave is under about 10 knots of wind shear. The shear is forecast to remain below 10 knots through Tuesday, and there is some favorable anticyclonic outflow at high levels, and 97L has a good chance of developing into a tropical depression by Tuesday. At that point, the future evolution of the storm depends strongly on how far north it is. If 97L moves northwest over Puerto Rico on Wednesday, as the GFDL and some of the global models predict, it may encounter a zone of high wind shear associated with the bottom part of a trough of low pressure positioned to the north of Puerto Rico. This shear should keep the storm from becoming a hurricane. If 97L stays on a more west-northwest track and penetrates into the Caribbean Sea south of Puerto Rico, as predicted by the simpler BAMM model, the storm is likely to encounter less shear, and could grow into a hurricane. Regardless, 97L will bring heavy rain and gusty winds to most of the Lesser Antilles Islands tonight through Tuesday.

Figure 1. Microwave image from 4:46 pm EDT today showing low-level spiral bands starting to form on the west side of 96L. Image credit: Navy/NRL.

Far Atlantic disturbance 96LA tropical wave "96L" in the far eastern Atlantic, about 650 miles southwest of the Cape Verdes Islands, has gotten more organized during the past 24 hours, as seen in the latest Satellite imagery. The circulation associated with the wave is unusually large. The storm will be a little slow to get going, since the storm is so far south. At the storm's current latitude--6 degrees north of the Equator--it cannot leverage the earth's spin very much to help spin up the huge circulation it has. Despite it's close proximity to the Equator, low-level spiral bands have already formed, as seen in recent microwave satellite images (Figure 1). The wave is under about 10 knots of wind shear. The shear is forecast to remain below 10 knots through Wednesday, and there is some favorable anticyclonic outflow at high levels. There is a good chance 96L will become a hurricane late this week, as forecast by the SHIPS intensity model. The Lesser Antilles Islands should anticipate the possibility that this will be a hurricane by the time it reaches the islands seven days from now, although it could miss to the north. It is possible 96L will encounter a zone of high wind shear beginning four days from now. The HWRF model develops 96L into a 55-mph tropical storm by Tuesday, then weakens the system the remainder of the week. The GFDL model does not develop 96L at all.

I'll edit this blog tonight to include the evening QuikSCAT pass, if it hits 96L. Otherwise, I'll be back Monday morning--and maybe I'll even talk about our one active storm, Jerry!

TD 10 spawns EF-1 tornado in Florida; new disturbance a threat to Texas and Louisiana

Tropical Depression Ten moved ashore last night over the Florida Panhandle, bringing rains of 1-5 inches over the region (Figure 1). The most serious weather associated with the depression occurred when a tornado ripped through Eustis, Florida at 11 pm Friday night. The EF-1 tornado had winds up to 105 mph, and damaged about 100 homes. The remnants of TD Ten are over southern Mississippi this morning, and additional severe weather or heavy rain is not expected.

Western Caribbean disturbance 94LAn area of disturbed weather in the western Caribbean between the Yucatan Peninsula and Jamaica is associated with a surface trough of low pressure. NHC designated this area "94L" this morning. Satellite loops show that the heavy thunderstorm activity has increased today in the region, but remains disorganized. A buoy in the region recorded sustained winds of 31 knots, gusting to 35 at 4:50 am EDT. The winds have since subsided to 20 knots. Cancun radar shows heavy rains have already moved ashore over the eastern Yucatan. This morning's QuikSCAT pass showed no signs of a circulation, and very little evidence of even a wind shift in the region. Thus, the earliest I expect 94L can become a tropical depression is Sunday afternoon. An Air Force Hurricane Hunter aircraft is scheduled to investigate 94L Sunday afternoon.

This disturbance will bring heavy rains to Belize, Cozumel, Cancun, and western Cuba today as it crosses the Yucatan Peninsula. Moisture streaming northwards from the disturbance will also cause locally heavy rains across the Florida Peninsula. Wind shear has dropped to about 10 knots over the disturbance, and the NOGAPS and GFS models predict this shear will stay low enough to allow a tropical depression to form on Sunday when 94L crosses into the Gulf of Mexico. By Monday afternoon, my best guess is that 94L will make landfall near the Texas/Louisiana border. That doesn't give it much time to organize into a tropical depression or tropical storm. Today's 12Z (8 am EDT) run of the GFDL model did not develop 94L. The 12Z SHIPS model developed it into a 45-mph tropical storm by Monday morning. Regardless, Texas and/or Louisiana can expect very heavy rains Monday and Tuesday from this system.

Elsewhere in the tropicsA few clumps of heavy thunderstorm activity exist along the Intertropical Convergence Zone (ITCZ), about 800-1200 miles east of the southernmost Lesser Antilles. This activity is moving west at 10-15 mph, and is very disorganized. Nevertheless, the region is under only about 10 knots of wind shear, so we will need to watch this area for development. A tropical wave near 6N, 23W, about 60 miles south of the Cape Verdes Islands off the coast of Africa, has some vigorous thunderstorm activity associated with it. This morning's 4:30 am EDT ASCAT pass showed a nearly complete circulation, and visible satellite images also show a fair bit of spin. This wave has the potential to develop into a tropical depression early next week as it moves westward at 15 mph.

I'll be traveling Sunday, and will not post a blog if the Western Caribbean disturbance fizzles. Otherwise, I'll post something late Sunday afternoon when the Hurricane Hunter mission sends back data.

Tropical Depression Ten slowly intensifying

Tropical Depression Ten has gradually been acquiring tropical characteristics today, and officially made the transition from a subtropical storm to a tropical storm at 2 pm. One could still argue that the storm is subtropical, since the satellite presentation still shows a large band of strong thunderstorms well removed from the center. However, TD 10 has developed some heavy thunderstorms right at its center, which is a key characteristic of a tropical system. This transition means that TD 10 is likely to intensify, and surface winds measured by the Hurricane Hunter aircraft have increased about 10% since this morning. An Air Force aircraft measured a 37 knot surface wind at 1:05 pm EDT, and a NOAA aircraft measured 38 knots at 1:35 pm EDT. It's a judgment call on NHC's part to decide when this slow increase in winds merits an upgrade of the system to Tropical Storm Jerry. The storm still has subtropical characteristics, and this will keep its intensification rate far slower than Hurricane Humberto's. I think a 50 mph tropical storm is the strongest we'll see TD 10 get.

Top surface winds measured at buoys in the Gulf of Mexico were all below 25 knots (29 mph) early this afternoon. Long range radar loops from the Florida Panhandle show a slow improvement in the organization of the low level spiral bands. Satellite loops show that heavy thunderstorm activity is rather limited but slowly increasing. Radar estimated precipitation over the Florida Panhandle (Figure 1) has been up to three inches.

Figure 1. Estimated rainfall from the Tallahassee, Florida radar.

Elsewhere in the tropicsAn area of disturbed weather in the western Caribbean between the Yucatan Peninsula and Jamaica is associated with a surface trough of low pressure. The counterclockwise flow of air around TD 10 is feeding moisture from this disturbance across Cuba and into South Florida today. This disturbance will bring heavy rains to Cozumel, Cancun, and western Cuba on Saturday as it crosses the Yucatan Peninsula. The disturbance is under 20 knots of wind shear at present, but the NOGAPS and GFS models predict this shear may fall enough on Sunday or Monday to allow some development to occur. Texas and Louisiana may get heavy rains from this system on Monday.

A non-tropical low pressure system about 1000 miles east of Bermuda is being watched by NHC for tropical development. SSTs beneath the low are 80 degrees F, which is right at the border where tropical storm formation can occur. This low is expected to move northeastwards out to sea 2-4 days from now.

Gulf of Mexico low nears tropical depression status

A large low pressure system (93L) over the Gulf of Mexico has gradually been acquiring tropical characteristics, but is in no hurry to intensify. Top surface winds measured at buoys in the Gulf of Mexico this morning have been 30 knots (35 mph), so this storm could technically qualify as a tropical depression if the current hurricane hunter mission finds a well-defined center of circulation. There are two Hurricane Hunters aircraft in the storm right now, and they have found several swaths of surface winds of 35 mph. Long range radar loops from the Florida Panhandle clearly show the storm's circulation, but the low level spiral bands are not well organized and are only slowly getting more organized. Satellite loops show a large, sloppy-looking storm. Storms that start off as large, non-tropical systems like 93L typically take several days to organize and become fully tropical, and 93L will not have time to become anything worse than a minimal tropical storm. The storm is capable of dumping some heavy rains along its path--radar estimated precipitation from the Tallahassee radar (Figure 1) were as high as three inches. As 93L becomes more tropical in nature, it will be able to generate higher rain amounts. But, with the storm expected to move inland by Saturday afternoon, it does not appear 93L has time to generate the kind of tropical rains that would make it a serious flood threat.

Figure 1. Estimated rainfall from the Tallahassee, Florida radar.

Elsewhere in the tropicsAn area of disturbed weather has developed in the western Caribbean between the Yucatan Peninsula and Jamaica, in association with a surface trough of low pressure. This disturbance will bring heavy rains to Cozumel and Cancun on Saturday as it crosses the Yucatan Peninsula. The region will need to be watched for development on Sunday as it crosses the Gulf of Mexico. The NOGAPS and GFS models predict an upper air pattern favorable for formation of a tropical depression over the Gulf of Mexico on Sunday and Monday. Heavy rains from this system may hit Louisiana and/or Texas on Monday.

A non-tropical low pressure system about 1000 miles east of Bermuda is being watched by NHC for tropical development. This low is expected to move northeastwards out to sea 2-4 days from now.

Figure 2. Current visible satellite image showing 93L near the Florida Panhandle (top of image) and the western Caribbean disturbance (bottom right of image).

I'll have an update late this afternoon when the next set of computer models runs are available.

Complicated Gulf of Mexico disturbance 93L primarily a rain threat

A very complicated weather situation over the Gulf of Mexico and Atlantic waters off the Southeast U.S. coast associated with a non-tropical low pressure system (93L), has brought heavy rains to Florida, Georgia, and South Carolina over the past 12 hours. A cold-cored upper level low pressure system a few hundred miles southwest of Tampa, Florida is primarily responsible for the the action. Late yesterday afternoon, a separate area of surface low pressure formed near Daytona Beach, bringing high surf and heavy rains of up to five inches along the Florida coast from Daytona to Jacksonville. This low moved inland over Florida, but the associated surge of moisture rotated northwards all the way to South Carolina. High surf warnings and coastal flood watches have been posted for Charleston, South Carolina today. This morning's QuikSCAT pass showed winds up to 50 mph well offshore of South Carolina. These winds have created a storm surge of up to two feet along the South Carolina coast. This second low pressure system was identified as "93L" by NHC beginning at 2 pm EDT yesterday. However, now that the low has weakened crossing the Florida Peninsula, the "93L" designation has been taken away from it, and attached to the upper level low pressure system in the Gulf of Mexico.

Recent Satellite loops and the Tampa Bay long range radar show that this non-tropical low pressure system is beginning to get more organized and is acquiring tropical characteristics. Substantial pressure falls are occurring at the surface underneath the upper level low, and this system is on its way to becoming a subtropical depression. A surface low pressure system vertically aligned with a cold-cored upper level low will usually take two or more days to make the transition to a warm-cored tropical storm. Rapid intensification cannot occur until the system is fully warm-core. Since landfall is expected Saturday between the Florida Panhandle and Southeast Louisiana, 93L probably does not have time to become fully tropical. If 93L makes landfall Saturday, it should not have winds stronger than about 55 mph. The GFDL, HWRF, and SHIPS intensity models all keep 93L's winds below 55 mph. If the storm spends an extra day over water and makes it to Texas, as the ECMWF model predicts, 93L could become fully tropical and make landfall as a strong tropical storm with 60-70 mph winds. However, there is plenty of dry air in the environment, and I don't think the storm will be able to intensify to a strong tropical storm. The primary threat from 93L will be heavy rain, and the northern Gulf Coast from the Florida Panhandle to Texas border can expect a soaking from this system.

The Air Force Hurricane Hunters are scheduled to investigate 93L this afternoon at 2pm EDT.

Figure 1. Current long range radar out of Tampa Bay, Florida.

Remains of IngridThe remains of Tropical Storm Ingrid are still active, triggering some heavy thunderstorm activity a few hundred miles north of the northernmost Lesser Antilles Islands. Puerto Rico long range radar and satellite loops show that this activity remains disorganized. Wind shear has dropped to about 10 knots today, and we will need to watch this area for development. However, the upper level winds are not in a particularly favorable configuration, and Ingrid's remains are so disorganized, that any development will be slow to occur. The remains of Ingrid are in a region of weak steering currents, and little movement is expected over the next 3-5 days.

Gulf of Mexico disturbance 93L less of a threat

Recent Satellite loops and the Tampa Bay long range radar show that a non-tropical low pressure system (93L) appears to be reforming off the southwest coast of Florida. This is an important shift, since it brings the surface low underneath the upper level low pressure system aloft--the same kind of situation one finds in ordinary non-tropical "cutoff lows". This is an unusual event in September over the Gulf of Mexico, and is good news for those potentially living in the path of 93L. A surface low pressure system vertically aligned with a cold-cored upper level low will usually take a day or two to make the transition to a warm-cored tropical storm. During this kind of transition, rapid strengthening is rare, and the chances of 93L ever reaching hurricane strength now appear dim. The latest (12Z, 8am EDT) intensity forecasts from the GFDL and HWRF computer models keep keep 93L below hurricane strength, as does the 18Z (2pm EDT) SHIPS intensity model. The HWRF model indicates that 93L will come ashore at tropical depression strength, and this is entirely possible.

The forecast tracks from the latest cycle of model runs all show a landfall between central Louisiana and Pensacola, Florida on Saturday morning. The exceptions are the NOGAPS and ECMWF models, which show a Sunday morning landfall near the Texas/Louisiana border.

Figure 1. Current long range radar out of Tampa Bay, Florida.

Secondary low develops off Cape CanaveralA secondary low pressure system developed late this afternoon just north of Cape Canaveral, Florida, and has moved inland. This second low is now being referred to as "93L" by NHC, not the low developing off the Southwest Florida coast. Both of these lows need to be tracked, however. The new low north of Cape Canaveral has generated some impressive pressure falls of up to 9 mb in the past 24 hours, and has brought rainfall amounts of five inches along the coast from Daytona to Jacksonville. Heavy rains will spread across northern Florida Thursday as this low tracks westward. If the low pops out over the Gulf of Mexico south of the Florida Panhandle, it could intensify into a tropical depression and bring very heavy rain to the Florida Panhandle.

The NOAA jet is scheduled to fly this evening and collect data to help with the Wednesday evening (00Z Thursday) model runs. The models should have a much better handle on both of these lows early Thursday morning. It will be interesting to see which low ends up being dominant.

Remains of IngridThe remains of Tropical Storm Ingrid are still kicking up some heavy thunderstorm activity a few hundred miles north of the northernmost Lesser Antilles Islands. There is not much spin evident on satellite loops, but wind shear has dropped from 30 knots yesterday to 10 knots today, and we will need to watch this area for development. Steering currents are weak in this region, and the remains of Ingrid will move little over the next 3-5 days.

I'll have an update Thursday morning. I haven't quite finished my wind shear tutorial yet, but I will post that as soon as I finish it.

Florida storm 93L a threat to the Gulf Coast

A non-tropical low pressure system (93L) developed yesterday afternoon off the southeast coast of Florida, and is bringing heavy rains to central Florida today. Animations of long-range radar out of Melbourne, Florida and satellite loops show a large, disorganized area of thunderstorms affecting the Florida Peninsula and adjacent waters. The surface center of circulation is right at the coast north of West Palm Beach. Water vapor satellite loops show that the upper level low pressure system that helped spawn 93L has moved off the southwest coast of Florida, and is headed westward across the Gulf of Mexico. This upper level low is bringing about 20 knots of wind shear over 93L, and pulling in some dry air from the northwest. As 93L traverses Florida today and tonight, it will bring heavy rains to the state. Radar estimated rainfall has been less than three inches thus far. No development of 93L is expected until the surface circulation emerges off the Gulf Coast of Florida Thursday.

Figure 1. Current long range radar out of Melbourne, Florida.

Once 93L emerges into the Gulf of Mexico Thursday, all of the models indicate the storm will intensify. This is a very complex forecast situation, since the storm is starting out with the cold core of an ordinary non-tropical low pressure system, and will transition to a warm-cored tropical storm. The transition to a warm core system will probably take at least a day. A storm undergoing such a process cannot intensify rapidly while this is occurring. This means that if 93L hits New Orleans Friday night/Saturday morning as the GFS and GFDL models are predicting, the storm will likely still be below hurricane strength--as predicted by the 8am EDT run of the SHIPS intensity model--or a minimal Category 1 hurricane--as predicted by this morning's 06Z run of the GFDL model. I think a tropical storm is more likely. Such a track would take it just north of the high heat content waters of the Loop Current in the central Gulf of Mexico (Figure 2). If 93L takes a more southerly track as the ECMWF and NOGAPS models predict, it will have an extra day over water, and more time to firmly establish a warm core. A warm core, fully tropical system is capable of must faster intensification rates. A more southerly track would also take the storm over the high heat content waters of the Loop Current, further aiding the transition to a warm core system. Texas could see a Category 1 or 2 hurricane on Sunday in this scenario. Slowing down the intensification will be the presence of plenty of dry air to the northwest, however, and a tropical storm may be all that Texas would see.

The NOAA jet is scheduled to fly this evening and collect data to help with the Wednesday evening (00Z Thursday) model runs. The first Air Force Hurricane Hunter mission is scheduled for Thursday afternoon.

Figure 2. Tropical Cyclone Heat Potential (TCHP) for September 18, 2007. TCHP is a measure of the total heat energy available in the ocean, and values greater than 90 kJ per square centimeter can trigger rapid intensification of tropical cyclones. The very high values of TCHP in the central Gulf of Mexico are associated with a warm ocean current known as the Loop Current. Image credit: NOAA/AOML.

Remains of IngridThe remains of Tropical Storm Ingrid are still kicking up some heavy thunderstorm activity a few hundred miles north of the northernmost Lesser Antilles Islands. There is still a bit a spin evident on satellite loops and this morning's QuikSCAT pass. Wind shear has dropped from 30 knots yesterday to 20 knots today, and is expected to decline below 10 knots by Thursday. We will need to watch this area for development. Ingrid's remains are moving slowly northwest. Steering currents are weak in the area, and the system will probably not move much over the next five days.

Typhoon Wipha whiffedTyphoon Wipha made landfall just south of China's most populous city, Shanghai, at 3 am local time this morning. Wipha weakened significantly to a Category 2 storm just before landfall, and then to a tropical storm as it passed west of Shanghai. Damage from Wipha's winds and rain was far less than originally feared.

Florida disturbance 93L getting more organized

An area of disturbed weather (labeled "93L" by NHC this afternoon) has become much more organized this afternoon. Animations of long-range radar out of Miami, Florida and satellite loops show that thunderstorm activity off the Florida coast is steadily increasing. A surface circulation has not yet fully formed, but has almost closed off just south of Grand Bahama Island, about 100 miles east of Fort Lauderdale, Florida. Winds at Settlement Point on Grand Bahama Island were blowing at 9 knots out of the east at 9am this morning, but shifted to northerly and now northeasterly, and have increased to 32 knots (37 mph), with gusts to 34 knots (39 mph, tropical storm force). Winds at Freeport on Grand Bahama Island have increased to 30 mph, with gusts as high as 37 mph.

Water vapor satellite loops show that an upper-level low pressure system has detached from its parent trough over northern Florida. This upper level low is moving southwest, creating more favorable upper-level winds over 93L. Wind shear over 93L has fallen from 30 knots to 20 knots in the past six hours. A subtropical depression could form before it moves over South Florida tonight or Wednesday morning. Since this system is not fully tropical and does not have a warm core, it is very unlikely that it will be able to rapidly intensify today, ala Humberto. Regardless, South Florida and the western Bahamas can expect heavy rains of 3-6 inches from this system. Radar estimated rainfall has been as high as three inches in some spots over the ocean thus far.

Figure 1. Current long range radar out of Miami, Florida.

Most of the computer models predict that a tropical or subtropical depression will form from 93L once it crosses into the Gulf of Mexico. The path such a storm might take is uncertain. A strong ridge of high pressure is setting up over the eastern half of the U.S., and is expected to remain anchored in place for at least ten days. This is the type of steering pattern we experienced during the Hurricane Season of 2005, and favors westward-moving storms. However, this steering pattern will be complicated by the presence of the upper-level low pressure system moving southwest over the Gulf of Mexico. This upper-level low will gradually weaken. Depending on the strength and movement of this low, the counterclockwise flow around the low could steer 93L on a northwesterly path towards Louisiana. This is the solution of the latest (12Z) runs of the UKMET, GFS, and GFDL models. The intensity such a storm may reach is also highly uncertain. The storm is starting off without a warm core, which will hamper intensification. Dry air to the west will also cause it problems. The SHIPS intensity model brings 93L to Category 1 hurricane strength by Saturday, and the GFDL predicts 93L will hit New Orleans as a strong tropical storm Friday night. The NOGAPS model predicts 93L will eventually dissipate over the southwestern Gulf, and the HWRF model does not develop 93L, and takes the disturbance to the central Louisiana coast on Saturday afternoon.

The NOAA jet is scheduled to fly Wednesday evening and collect data to help with the Wednesday evening (00Z) model runs. The first low-level Hurricane Hunter mission is scheduled for Thursday afternoon.

Remains of IngridThe remains of Tropical Storm Ingrid are still kicking up some heavy thunderstorm activity a few hundred miles north of the northernmost Lesser Antilles Islands. There is still a bit a spin evident on satellite loops and this morning's QuikSCAT pass. Wind shear is a hefty 30 knots, and is expected to gradually decline to 10 knots by Wednesday night. If there's anything left of Ingrid then, we will need to watch this area for regeneration.

Tropical wave in the mid-AtlanticA tropical wave about 750 miles east of the Lesser Antilles Islands is moving west at 10-15 mph. Thunderstorm activity associated with this wave has diminished substantially since yesterday, and any development should be very slow to occur. Wind shear is about 10 knots, and is expected to remain below 15 knots over the next 2-3 days. None of the computer models develop this wave into a tropical depression.

Typhoon Wipha takes aim at China's most populous cityTyphoon Wipha, a formidable Category 4 storm, is making landfall just south of China's most populous city, Shanghai. About 14.5 million people live in the city. Over 1 million people have been evacuated so far. Wipha must pass over about 50 miles of land before reaching Shanghai, and will probably be a strong tropical storm or Category 1 hurricane when it passes over or just west of the city. Rainfall amounts of up to seven inches in 24 hours are expected along the path of Wipha, which will cause major flooding problems. Fujian province on China's southeastern coast has sent out 1.41 million text messages to warn the public of the upcoming typhoon, the local flood control headquarters said. The Women's World Cup soccer tournament is going on in China, and the U.S. is supposed to play their final group game tomorrow in Shanghai (8pm Tuesday night China time, 8am EST). World Cup organizers are trying to change the game time to get the game in (and then the teams out of Shanghai) before the storm hits.

Yesterday, Wipha briefly intensified into a super typhoon packing 150 mph winds as it brushed Taiwan. One person was killed there. Wipha is a woman's name in the Thai language.

An area of disturbed weather has developed off the east coast of Florida, in association with a tropical wave interacting with a trough of low pressure. This disturbance has been labeled "93L" by NHC this afternoon. Animations of long-range radar out of Melbourne, Florida and satellite loops show that thunderstorm activity off the Florida coast is increasing, but remains disorganized. Wind shear over the disturbance has fallen from 30 knots to 20 knots today, and is expected to fall below 10 knots by Thursday. The disturbance is moving westward, and will bring heavy rain to Florida and the Bahamas today through Thursday.

Figure 1. Current long range radar out of Melbourne, Florida.

The four reliable computer models for forecasting tropical cyclone genesis all predict that once this area of disturbed weather crosses Florida and emerges into the eastern Gulf of Mexico on Thursday, wind shear will drop enough to allow a tropical depression to form. The path such a storm might take is highly uncertain, and the models have diverged significantly since yesterday's runs. A strong ridge of high pressure is setting up over the eastern half of the U.S., and is expected to remain anchored in place for at least ten days. This is the type of steering pattern we experienced during the Hurricane Season of 2005, and favors westward-moving storms. This ridge will act to steer any developing storm in the Gulf of Mexico towards the west, to the Texas coast, or even west-southwest, to the northeastern mainland Mexico. However, this steering pattern will be complicated by the presence of an upper-level low pressure system over the Gulf of Mexico. This upper-level low is currently forming over northern Florida, and is expected to drift southwestward or westward over the Gulf later this week, and gradually weaken. Depending on the strength and movement of this low, a storm that forms in the eastern Gulf could be steered anywhere from the Florida Panhandle to northeastern Mexico. One model, the NOGAPS, predicts that the storm will move west-southwest into the Bay of Campeche (southwestern Gulf of Mexico), and stall out for many days. The UKMET, GFS, and ECMFW models foresee that the storm will make landfall in Texas or Louisiana Sunday or Monday. The intensity such a storm may reach is also highly uncertain. The NOGAPS model predicts 93L will eventually dissipate over the southwestern Gulf, while the SHIPS intensity model brings 93L to Category 1 hurricane strength by Saturday. All residents along the Gulf of Mexico coast need to consider what actions they might need to take if a hurricane develops in the Gulf this weekend. The highest danger region is from New Orleans westward to Brownsville, Texas. A hurricane hunter airplane is on call for Thursday afternoon.

Remains of IngridThe remains of Tropical Storm Ingrid are still kicking up some heavy thunderstorm activity a few hundred miles north of the northernmost Lesser Antilles Islands. There is still a bit a spin evident on satellite loops and this morning's QuikSCAT pass. Wind shear is a hefty 30 knots, and is expected to gradually decline to 10 knots by Wednesday night. If there's anything left of Ingrid then, we will need to watch this area for regeneration.

Tropical wave in the mid-AtlanticA tropical wave about 750 miles east of the Lesser Antilles Islands is moving west at 10-15 mph. Thunderstorm activity associated with this wave has diminished substantially since yesterday, and any development should be very slow to occur. Wind shear is about 10 knots, and is expected to remain below 15 knots over the next 2-3 days. None of the computer models develop this wave into a tropical depression.

Typhoon Wipha takes aim at China's most populous cityTyphoon Wipha, a formidable Category 4 storm, is poised to make landfall tomorrow just south of China's most populous city, Shanghai. About 14.5 million people live in the city. Over 1 million people have been evacuated so far. Wipha must pass over about 50 miles of land before reaching Shanghai, and will probably be a strong tropical storm or Category 1 hurricane when it passes over or just west of the city. Rainfall amounts of up to seven inches in 24 hours are expected along the path of Wipha, which will cause major flooding problems. Fujian province on China's southeastern coast has sent out 1.41 million text messages to warn the public of the upcoming typhoon, the local flood control headquarters said. The Women's World Cup soccer tournament is going on in China, and the U.S. is supposed to play their final group game tomorrow in Shanghai (8pm Tuesday night China time, 8am EST). World Cup organizers are trying to change the game time to get the game in (and then the teams out of Shanghai) before the storm hits.

Yesterday, Wipha briefly intensified into a super typhoon packing 150 mph winds as it brushed Taiwan. One person was killed there. Wipha is a woman's name in the Thai language.

Ingrid dies; possible Gulf of Mexico storm this week

Tropical Depression Ingrid finally succumbed to a long period of 20-30 knots of wind shear that lasted three days. There is still a bit of spin left to Ingrid's remains, and some scattered thunderstorm activity. However, with wind shear still 20-30 knots, and expected to stay 20-30 knots for at least another day, it will be difficult for Ingrid's remains to regenerate into a tropical storm again.

Gulf of Mexico and Western Caribbean development?The four reliable computer models for forecasting tropical cyclone genesis continue to predict that a tropical storm will pop up in the Western Caribbean or Gulf of Mexico by Thursday. The forecasted storm is expected to move westward into Texas by Sunday. However, steering currents will be complicated by the expected formation of an upper-level low pressure system over the Gulf of Mexico later this week. The Yucatan Peninsula, western Cuba, northern Gulf Coast, and Florida Gulf Coast could also be at risk.

The seed for the formation of such a storm could come from a tropical wave in the Central Caribbean that was kicking up some heavy thunderstorm activity yesterday. This activity has quieted down today. Another possibility is that an old frontal zone stretching from the Carolinas southwards along the U.S. East Coast and across northern Florida could spawn something. I give a 40% chance a tropical storm will form in the Western Caribbean or Gulf of Mexico this week. NHC had a hurricane hunter aircraft on call to fly today in case something popped up, but this flight was canceled.

Tropical wave in the mid-AtlanticA tropical wave near 12N, 43W, midway between the Lesser Antilles Islands and the coast of Africa, is moving west at 10-15 mph. This wave has some disorganized thunderstorm activity, and some slow development is possible over the next 2-3 days. Wind shear is about 10 knots, and is expected to remain below 15 knots over the next 2-3 days. The UKMET model is the only reliable model developing this wave into a tropical depression. The Hurricane Hunters are on call to investigate this system on Wednesday, when it should be about 500 miles east of the Lesser Antilles Islands.

No change to Ingrid; possible Gulf of Mexico storm this week

Tropical Depression Ingrid has shown little change over the past day. The depression is slowly tracking west-northwest in the teeth of strong upper-level westerly winds, which are creating about 25 knots of wind shear over the storm. Satellite loops of Ingrid show that the storm has managed to keep a repectable amount of heavy thunderstorm activity near its center, but these thunderstorms are disorganized. This morning's QuikSCAT pass showed a well-formed circulation, but top winds of only 25 mph.

The wind shear is expected to remain around 20-30 knots Sunday and Monday, then decline to 10 knots on Tuesday and Wednesday. It is questionable whether Ingrid can survive the high sustained levels of shear expected for the next two days. It is unclear whether Ingrid (or its remains) will recurve out to sea or be forced westward towards the U.S. in the 6-10 day time frame, but residents of Bermuda and the U.S. East Coast should continue to watch this system--it could reform if it is destroyed by wind shear.

Gulf of Mexico storm possible this weekThe four reliable computer models for forecasting genesis of tropical cyclones have been very busy the past few runs cooking up some nasty storms in the Western Caribbean and Gulf of Mexico for the coming week. Neither the timing nor the location of these hypothetical storms has been consistent. However, the models are insistent enough that something might happen, that I believe there is about a 40% chance we'll see a tropical storm in the Gulf of Mexico by week's end. A few possibilities, from this morning's model runs:

NOGAPS: A tropical storm forms in the Western Caribbean Tuesday, and moves north, hitting South Florida Friday.

UKMET, ECMWF, and GFS: A tropical storm forms in the central Gulf of Mexico Thursday and moves west, hitting Texas on Saturday.

The seed for formation of a tropical storm in the Western Caribbean would be one of the tropical waves from Africa that are parading across the Atlantic. A Gulf of Mexico storm could get spawned from a tropical wave, or from an old frontal zone stretching from the Carolinas southwards along the U.S. East Coast then across northern Florida.

Tropical wave in the mid-AtlanticA tropical wave near 9N, 39W, midway between the Lesser Antilles Islands and the coast of Africa, is moving west at 10-15 mph. This wave has shown an increase in heavy thunderstorm activity over the past day, and some slow development is possible over the next 2-3 days. Wind shear has increased some over the wave, to 10-20 knots, and is expected to remain below 20 knots over the next 2-3 days.

Ingrid disintigrating under high wind shear

Tropical Storm Ingrid is slowly disintegrating in the face of strong upper-level westerly winds, which are creating about 30 knots of wind shear over the storm. Satellite loops of Ingrid show that the shear has removed almost all of Ingrid's heavy thunderstorm activity. This wind shear is expected to remain around 30 knots through Sunday, then gradually decline to 10 knots by Monday. It is questionable whether Ingrid can survive such high sustained levels of shear, and there is at least a 60% chance the storm will be destroyed. Even in the event the storm is destroyed, it could regenerate early next week and threaten Bermuda. It is unclear at this time whether Ingrid (or its remains) will recurve out to sea or be forced westward towards the U.S. in the 6-10 day time frame.

Elsewhere in the tropicsThe GFS, ECMWF, and UKMET models all suggest a tropical depression may form in the Western Caribbean on Wednesday and move northwards into the Gulf of Mexico or over Florida.

A tropical wave a few hundred miles west-southwest of the Cape Verdes Islands off the coast of Africa is moving west at 10-15 mph. This wave shows no signs of development, but is under only 10 knots of wind shear. Shear is expected to remain below 10 knots over the next 2-3 days over the wave, which may allow it to develop.

Ingrid is born; Humberto and Felix--a sign of climate change?

Tropical Storm Ingrid finally managed to get organized enough last night to earn a name. Ingrid continues to struggle with strong upper-level westerly winds, which are creating about 15 knots of wind shear over the storm. Satellite loops of Ingrid show that the shear is keeping most of Ingrid's heavy thunderstorm activity pushed to the storm's east side. There is some weak upper level outflow to the north, and one low-level spiral band forming on the storm's west side. The NOAA Hurricane Hunters are in the storm flying a research mission, and have measured top surface winds of 59 mph with their SFMR instrument.

Figure 1. Microwave image of Ingrid at 7:28am EDT. Ingrid has just one clump of heavy thunderstorm activity near the center, and one spiral bands forming on the west side. Image credit: Navy Research Lab, Monterey.

Wind shear is forecast to remain below 15 knots over Ingrid until Saturday afternoon, then steadily rise to values near 30 knots. Ingrid will probably increase in strength over the next 24 hours, then weaken. It is possible that the shear will destroy Ingrid by the time the storm makes its closest approach to the Lesser Antilles Islands on Tuesday. The exact track of Ingrid as it approaches the islands depends on how strong the storm gets over the next 24 hours--a stronger storm will move more to the northwest, since it will "feel" upper level winds that will pull it farther north. As a result, the 5-day track forecast for Ingrid has higher uncertainty than usual, and it is not a sure thing that Ingrid will miss the Lesser Antilles. The HWRF, GFDL, and SHIPS intensity models all forecast that Ingrid will survive the shear and will be a weak tropical storm when making its closest approach past the islands.

If Ingrid does survive past Tuesday, it may encounter a region of lower wind shear 6-10 days from now that will allow it to re-intensify. A large ridge of high pressure is forecast to dominate the region late next week, which should force Ingrid on a west to northwest track towards the U.S. Depending upon how far north the storm is at that point, it may represent a threat to either Bermuda or the U.S. East Coast. The GFS model forecasts that the next major trough of low pressure capable of recurving Ingrid out to sea will not arrive until Saturday September 22. Stay tuned.

HumbertoThe surprise hurricane, Humberto, turned out not to be the flood-making threat originally feared. While the storm was able to dump 10-14 inches of rain over Jefferson County, Texas, Humberto's rains have mostly been under seven inches (Figure 2). Only 1-2 more inches are expected along its path today in Mississippi, Alabama, and Tennessee. All of these states are suffering severe drought, so Humberto is a welcome visitor.

Figure 2. Total rainfall from Humberto as estimated by radar.

Humberto, Felix, and Dean--a sign of climate change?Many people have asked me if the fact that we've had two record-breaking rapidly intensifying storms this year--Felix and now Humberto--imply that climate change might be affecting Atlantic hurricanes for the worse. It's also very odd that we've had eight Category 5 hurricanes in the past five years, and two landfalling Category 5 hurricanes this year. That's a lot of Cat 5 activity. So, let's look at the facts and see what we can learn.

If one considers instead the fastest intensification time from tropical depression strength to hurricane strength, Humberto has some company--Hurricane Blanche of 1969 did it in 12 hours, and Hurricane Alberto of 1982 did it in 18 hours. However, these storms spent two and three advisories, respectively, at tropical depression strength, and thus spent more time getting their act together than Humberto did. Thus, Humberto is unique in the Atlantic hurricane record.

Since 1970, Hurricane Felix of 2007 holds the record for fastest intensification from the first advisory to a Category 5 hurricane. It took Felix just 54 hours to accomplish the feat. Hurricane Camille of 1969 also took 54 hours to do so, but the first advisory put Camille as a 60 mph tropical storm. It is likely that Camille would have been classified as a tropical depression earlier had reliable satellite imagery been available.

Hurricane Ethel of 1960 holds the pre-1970 record for fastest intensification from the first advisory to a hurricane. Ethel strengthened from a 45-mph tropical storm to a 85 mph Category 1 hurricane in just 6 hours. We don't know when Ethel started as a tropical depression, since this was before the satellite era. Ethel also holds the record for quickest intensification from the first advisory to a Category 5 hurricane--it took Ethel just 18 hours. This all occurred while Ethel approached landfall on the Mississippi coast. Luckily, the storm fell apart just as quickly, weakening from a Category 5 hurricane to a tropical storm in the 12 hours prior to landfall.

The case of Ethel has not yet undergone rigorous review by the NHC committee that is quality checking and revising the entire Atlantic hurricane database. It is possible that some revisions may be made to these records, as the storm's wild swings in intensification seem rather extreme. The minimum pressure measured by the Hurricane Hunters in Ethel was 972 mb--more typical of a Category 2 hurricane than a Category 5. Still, there is little question that Ethel intensified remarkably fast, and we would be in big trouble if another Ethel formed by the coast and didn't fall apart before landfall.

Hurricane Wilma of 2005 holds the record for fastest intensification with respect to pressure by an Atlantic hurricane. Wilma's pressure dropped 97 millibars in 24 hours as it went from a tropical storm to a Category 5 hurricane. The previous record holder was Hurricane Gilbert (1988), which dropped 72 mb in 24 hours. Wilma's pressure fell 54 mb over six hours, beating Hurricane Beulah's drop of 38 mb in six hours in 1967, and Wilma's 12-hour pressure fall of 83 mb beat the old 12-hour pressure fall record of 48 mb set by Hurricane Allen in 1980.

No scientist has published a paper linking rapid hurricane intensification rates with global warming. While the cases of Humberto and Felix are certainly unique, the year 1969 also had two storms that were very similar in their intensification rates. A quick look I did at historical intensification rates don't show any noticeable trends, and I think that the rapid intensification rates of Felix, Humberto, and Wilma the past three years are not far enough outside the statistical norms that we need to invoke climate change as an explanation. Still, it does leave one wondering, and climate change could be affecting hurricane intensification rates.

Category 5 recordsSince reliable record keeping began in 1944, there have been 27 Category 5 hurricanes. It is possible that a few Cat 4's that should have been Cat 5's were missed, but I'm guessing this number is at most 10% of the total--two or three. Only ten hurricanes have made landfall at Category 5 strength. Two of those landfalls have occurred this year--the only year that has happened. Thus, the fact we've had 20% of all Category 5 landfalls on record in the same year is truly exceptional. The fact that they both occurred in the Western Caribbean back to back is not that surprising, since the Western Caribbean has the very high heat content waters one needs to fuel Category 5 hurricanes, and the landfalls occurred during the peak part of hurricane season.

This year is the fourth year multiple Cat 5's have occurred--see Wikipedia's Category 5 list to see the details. We've now had six Cat 5's in the past three years, and eight in the past five years. Is this an indication climate change is at work? Well, we did have back-to-back years with two Cat 5's each (1960 and 1961), so one can still argue that the Cat 5 activity of recent years is a statistical abnormality. In addition, recent work done studying sediment deposits indicates that intense hurricanes have gone through cycles lasting hundreds or even thousands of years long. Periods of high Category 5 activity similar to that observed the past five years could well have occurred in the distant past. Still, some very good hurricane scientists have begun presenting evidence that climate change may be increasing both the frequency and intensity of hurricanes in the Atlantic. It is possible that climate change may be partially responsible for the recent spate of Cat 5's and rapidly intensifying storms. Climate change is significantly affecting weather patterns worldwide, and must be influencing hurricanes. Unfortunately, we don't have a long enough or high enough quality data record of Atlantic hurricanes to accurately judge how much of an impact this might be. Furthermore, it's not clear why the Atlantic Ocean would be the most strongly affected--we see little evidence that climate change is creating stronger hurricanes in the other ocean basins. But, the events of 2005 and again this year leave me concerned. Eight Cat 5's in five years is an awful lot of severe storms in such a short period. Climate change may be indeed be changing Atlantic hurricanes for the worse.

Hurricane Humberto surprises Texas and Louisiana

A surprise Hurricane Humberto ripped into Texas near the Louisiana border this morning, bringing winds of 85 mph and torrential rains to the coast. Humberto didn't even exist yesterday morning, and grew from a tropical depression at 11am EDT to a hurricane just 14 hours later. As the hurricane intensified, it grew in size and height, allowing the storm to respond to upper-level winds that changed its course to a north-northeastly path. This allowed Humberto to stay over water parallel to the coast for the crucial hours needed for it to reach hurricane strength. Storms like Humberto give us the sobering reminder that as much as hurricane forecasting has improved in recent years, there is still much we do not understand--particularly in regards to intensity forecasting. If Humberto had had another 12-24 hours over water, it could have been a major hurricane that would have hit without enough time to evacuate those at risk.

Figure 1. Hurricane Humberto just after landfall as seen by the Lake Charles, LA radar.

Port Arthur, TX, on the border with Louisiana, has taken a heavy hit from Humberto. A portion of an apartment complex's roof was ripped off by the storm's winds, and fell on four vehicles. Widespread power outages were reported, with heavy flooding. A Personal Weather Station in downtown Port Arthur recorded a rainfall rate of over 2 inches per hour before failing at 3:30am EDT; a second station in the city recorded a minimum pressure of 989.7 mb before it, too failed. Nearby Beaumont, TX recorded sustained winds of 54 mph, gusting to 84 mph, at 5am EDT. On the coast where Humberto came ashore, McFadden wildlife Refuge measured sustained winds of 60 mph with a gust to 75 mph at 3:35am EDT. Winds of 67 mph gusting to 76 were recorded at nearby Sabine Pass. A crane at a work site at Sabine Pass recorded a wind gust of 118 mph at a height of 40 feet.

Figure 2. Current rainfall estimated by the Lake Charles radar.

While some moderate wind damage will affect Port Arthur and surrounding areas, and two tornadoes were reported near Galveston, the main threat from Humberto remains its rains. Estimated rain from the Lake Charles radar (Figure 2) show that amounts in excess of ten inches have fallen near Beaumont, with some higher amounts on the Louisiana coast over an unpopulated swampy area. Rains in excess of ten inches may fall along the path of Humberto as it plows through central Louisiana and into Mississippi. The storm may stall out and die over Mississippi, over the weekend, making for a dangerous flooding situation there. Hopefully, Humberto's rains will make it to Alabama, which is suffering its worst drought on record.

Tropical Depression EightTropical Depression Eight is near tropical storm strength. This morning's QuikSCAT pass showed a circular, well-defined circulation with a believable 40 mph wind reading near the center. However, recent Satellite loops of TD 8 show that wind shear has picked up this morning, completely exposing the low level circulation center. The wind shear will need to die down and allow heavy thunderstorm activity to build near the center before TD 8 will get named.

Wind shear is about 10 knots over TD 8, and is forecast to remain below 10 knots through Saturday morning, which should allow TD 8 time to grow to a modest tropical storm. Later on Saturday, the models now unanimously agree that TD 8 will encounter hostile upper-level westerly winds. The high levels of wind shear these upper-level winds will bring are likely to seriously weaken the storm, and may be able to tear it apart. It appears likely that TD 8 will pass north of the Lesser Antilles Islands, sparing them its strongest winds. Should TD 8 survive past Monday, it may represent a threat to the U.S. East Coast.

The NOAA Hurricane Hunters are scheduled to fly a research mission into TD 8 tonight.

Humberto's winds rising as it approaches landfall

The winds of Tropical Storm Humberto are increasing as the storm closes in on the Texas coast east of Galveston. The Hurricane Hunters in their 6:34pm EDT center fix reported a pressure of 998 mb, 7 mb lower than the estimated pressure from the 2 pm EDT advisory from NHC. The pressure was also 998 mb at 7:03pm, indicating that Humberto was taking a break from its intensification spurt. The drop in pressure has driven some stronger winds. The SFMR instrument on the aircraft measured one spot of 67 mph winds at 7:26 pm EDT, 59 mph at 7:08pm EDT, and 61 mph at 6:30pm EDT. These winds were judged as being too isolated to be representative of Humberto's large scale wind field by NHC, which kept Humberto's winds at 50 mph for the 7pm EDT advisory. The storm's relatively high central pressure of 998 mb is reasonable justification for this decision. The Hurricane Hunter airplane has now left Humberto, and a new plane is due to arrive at 2 am EDT.

Radar animations from Houston show a well-defined circulation, with plenty of low-level spiral bands wrapping around the center. The region of winds above 35 knots (40 mph) as estimated by the Houston Doppler radar have shown a steady increase, and there are now a few spots of 50 knot winds (58 mph) in the right front quadrant of Humberto. (keep in mind that due to the curvature of the earth, these winds are measured at an altitude of roughly 2,000 feet 50 miles from the radar). Radar shows that Humberto has not yet begun building an eyewall. With a pressure of 998 mb and only about six hours left before landfall, it is unlikely Humberto will have time to become a hurricane.

Figure 1. Latest radar image from Houston, Texas.

Since Humberto is slow moving, it has the potential to drop rain amounts in excess of ten inches along the Texas and Louisiana coasts over the next two days. It now appears that the Houston area will escape the worst of these rains, which will be concentrated more towards the Texas/Louisiana border region. These rains may cause widespread destructive flooding. Rainfall amounts of 2-3 inches have already occurred along the coast, from Freeport to just beyond the Louisiana border, as estimated by radar. Freeport may have had as much a five inches already.

Humberto will bring serious flooding to Texas and Louisiana; TD 8 strengthening in the Atlantic

Tropical Storm Humberto is steadily gaining strength as it closes in on the Texas coast near Freeport. Radar animations from Houston show a well-defined circulation, with plenty of low-level spiral bands wrapping around the center. Humberto is organizing quickly, and it is a good thing it has less than 12 hours remaining over water. Despite the rather intimidating appearance of Humberto on radar, the winds have not shown a major increase yet. A buoy 70 miles south of Freeport has measured sustained winds of 25 mph since 8:50am EDT. The region of winds above 35 knots (40 mph) as estimated by the Houston Doppler radar have shown a steady increase, doubling in area over the past two hours (keep in mind that due to the curvature of the earth, these winds are measured at an altitude of roughly 2,000 feet 50 miles from the radar). Top winds reported by the Hurricane Hunters in their 2:55pm EDT center fix were 38 knots at flight level, which corresponds to about 35 mph at the surface. They measured a pressure of 1001 mb, 4 mb lower than the estimated pressure from the 2pm EDT advisory from NHC. Winds from the SFMR instrument on the aircraft were 35 knots (40 mph) in several locations, with one spot of 51 mph winds.

Figure 1. Latest radar image from Houston, Texas.

Wind shear is a low 5-10 knots, and Humberto has time to develop into a 50-55 mph tropical storm before it makes landfall tonight. Since this system is slow moving, it has the potential to drop rain amounts in excess of ten inches along the Texas and Louisiana coasts--including the Houston metropolitan area--over the next two days. These rains may cause widespread destructive flooding. Rainfall amounts of 2-3 inches have already occurred along the coast, from Galveston to just beyond the Louisiana border, as estimated by radar.

Tropical Depression EightTropical Depression Eight formed this morning, and has the potential to grow into a hurricane in the next 3-4 days. This morning's 8:18am EDT pass of the European ASCAT satellite captured the circulation of TD 8 nicely, and showed that it was still somewhat elliptical, which will slow down intensification until the circulation grows more circular. ASCAT estimated winds up to 25 knots (30 mph) on the northwest side of TD 8. Satellite loops of TD 8 have shown a slow but steady improvement in organization since early this morning. A large area of thunderstorms reach high in the atmosphere (as evidenced by very cold cloud tops on infrared satellite imagery), an upper-level outflow channel has opened to the south, and some low-level spiral banding is now apparent. Several satellite intensity estimates put TD 8 at tropical storm strength already, and I expect that this will be a tropical storm later today.

Wind shear is about 10 knots over TD 8, and is forecast to remain below 10 knots through Saturday morning. This should allow TD 8 the opportunity to grow to a Category 1 hurricane by Saturday, as predicted by the latest (12Z, 8am EDT) rund of the GFDL and HWRF intensity models. By Saturday afternoon, the models have different solutions on what will happen to TD 8. An upper-level trough will lie north of the Lesser Antilles at that time, and may bring hostile wind shear to TD 8. The GFDL model predicts that this shear will be strong enough to reduce TD 8 to a weak tropical storm. The HWRF model disagrees, and keeps the storm at Category 1 hurricane strength. The SHIPS intensity model says the shear will increase to about 15 knots, which would slow but perhaps not completely halt intensification. The farther north the storm goes, the better--wind shear will be stronger to the north.

The NOAA Hurricane Hunters are scheduled to fly a research mission into TD 8 Thursday evening.

Two tropical depressions may form today

The area of disturbed weather 100 miles south of Freeport, Texas (90L), has become well-organized this morning, and a tropical depression appears to be forming. NHC put out this special advisory at 8:45am EDT today:Satellite and NWS radar observations indicate that the area of low pressure in the northwestern Gulf of Mexico is becoming better organized this morning. This system could become a tropical depression later today...and an Air Force Reserve unit reconnaissance aircraft will investigate the area early this afternoon. The low is moving slowly north-northwestward...and regardless of whether or not it becomes a tropical cyclone...heavy rains are expected to spread across southeastern Texas and Louisiana over the next couple of days. For information specific to your area...please consult statements issued by your local NWS forecast office.

Figure 1. Latest long-range radar image from Houston, Texas.

Long range radar out of Houston (Figure 1) shows a well-defined surface circulation exists about 100 miles south of Freeport, with some low-level spiral bands starting to get organized. A ship just east of the center measured 40 knot (46 mph) winds at 8am EDT, but this is likely a strong downburst wind from a thunderstorm, and is not representative of the larger-scale winds. A buoy 70 miles south of Freeport measured sustained winds of 25 mph at 8:50am EDT. Wind shear is a low 5-10 knots, and 90L does have time to develop into a tropical storm before it makes landfall tonight or Thursday along the mid or upper Texas coast. Since this system is very slow moving, it has the potential to drop rain amounts in excess of ten inches along the Texas and Louisiana coasts--including the Houston metropolitan area--over the next two days. These rains may cause serious widespread flooding. Rainfall amounts of 2-3 inches have already occurred along the coast, from Freeport to the Louisiana border, as estimated by radar. The Hurricane Hunters are scheduled to investigate this storm at 2pm EDT this afternoon.

91L could become a tropical depression todayA strong tropical wave near 12.5N 44W, 1100 miles east of the Lesser Antilles Islands, is headed west-northwest at 10 mph. This system (91L) may develop into a tropical depression today, and has the potential to become a large and dangerous major hurricane next week. This morning's QuikSCAT pass captured only the western side of the circulation, but did have some believable winds of 30-35 knots (35-40 mph). Last night's QuikSCAT pass showed an elliptical but better-defined circulation. The circulation looked more circular and better-defined in a pass from the European ASCAT satellite a few hours after that. A circular motion of winds around the center is much more conducive for strengthening than an elliptical circulation. Satellite loops show considerable improvement in organization has occurred since yesterday--a large area of thunderstorms that reach high in the atmosphere (as evidenced by very cold cloud tops on infrared satellite imagery) has formed near the circulation center. An upper-level outflow channel has opened to the south, and some low-level spiral banding is now apparent, particularly to the north. These spiral bands are also apparent on the latest microwave satellite imagery (Figure 1). Wind shear has dropped below 10 knots, and with warm 28 C waters underneath, 91L should become a tropical depression later today.

Figure 2. Microwave satellite image of 91L. Note the long spiral band forming to the north of the clump of heavy thunderstorms that lie near the center of 91L. Image credit: Navy Research Lab, Monterey.

Shear is forecast to remain below 10 knots for the next 3-4 days, which should allow 91L the opportunity to grow to a Category 1 hurricane or stronger before it reaches the Lesser Antilles Islands. However, the SHIPS, GFDL, and HWRF intensity models all keep 91L below hurricane strength for the next five days. I think this is unrealistic, given the favorable conditions for strengthening present. The big question concerns the track--there is a good chance that 91L will miss the Lesser Antilles, as the current steering currents favor a more northwesterly track for the storm over the coming days. The system has already moved north of Barbados' latitude, and the southern Lesser Antilles Islands are unlikely to receive a direct hit from 91L. Most of the models indicate a forward speed near 10-15 mph, which would bring 91L to the northern islands Monday. The U.S. East Coast may be at risk from this storm ten or so days from now, but it is far too early to speculate on the chance of this occurring, or what region might be most at risk.

The NOAA Hurricane Hunters may fly a research mission into 91L Thursday evening.

I'll have an update later today if either system develops into a tropical depression.

September hurricane outlook; Bill Proenza returns to his old job

A strong tropical wave near 11N 43W, halfway between Africa and the Lesser Antilles Islands, is headed west-northwest at 10-15 mph. This system (91L) has the potential to develop into a tropical depression later this week. This morning's QuikSCAT pass continued to showed a broad, elongated circulation and top winds of 25 mph. Satellite loops show no improvement in organization has occurred since yesterday--91L consists of some disorganized clumps of heavy thunderstorm activity. The disturbance is under about 10-15 knots of wind shear. Shear is forecast to remain near 15 knots today and Wednesday, then drop below 5 knots on Thursday. I expect this will allow 91L to develop into a tropical depression on Thursday. The HWRF brings it to a Category 3 hurricane by Sunday, at a position near 19N 58W, about 500 miles east-northeast of Puerto Rico. This is too aggressive an intensification rate, but I expect 91L will be at least a strong tropical storm by Sunday. The 06Z run of the GFDL model is more believable, making 91L a 55 mph tropical storm about 800 miles east of Puerto Rico on Sunday. This storm is definitely a threat to the Lesser Antilles Islands. It is too early to say if the northern islands are more at risk, as the current model runs are indicating. The system may represent a threat to the U.S. East Coast ten or more days from now, but there is no way to judge the likelihood of this.

Gulf of Mexico The area of disturbed weather in the southern Gulf of Mexico has diminished overnight, but does have the potential to grow again today before moving ashore over Texas and northern Mexico tonight and Wednesday morning, bringing heavy rain. New thunderstorm activity building off the Gulf Coast of Florida is very disorganized and is not a threat to develop.

Rest of September outlookWe're halfway. September 10 marks the peak of hurricane season in the Atlantic, and the frequency of hurricanes and tropical storms starts to decline now as water temperatures begin to cool and wind shear begins to rise. However, more activity typically occurs after September 10 than before, due to the fact that the ocean takes a long time to cool off. Given that we've already had 7 named storms so far this year, another 7 or 8 named storms are probably in order. The latest Dr. Bill Gray/Phil Klotzbach monthly forecast calls for 8 more named storms, with 5 of them hurricanes--two of these being intense hurricanes. They anticipate a later than usual end to hurricane season, due to the cooler than average conditions in the Eastern Pacific that might signal the beginning of a La Ni�a event. La Ni�a events typically bring lower than average wind shear to the Atlantic. Since high wind shear is usually what brings an end to hurricane season (this was certainly the case last year), development of a La Ni�a event over the next few months should act to prolong this year's hurricane season.

Figure 1. Average Atlantic hurricane activity.

Sea Surface Temperatures (Figure 2) are near average to 0.5 �C above average over much of the Main Development Region (MDR) for hurricanes (10� N to 20� N, from Africa to Central America). While this is nothing like the record SSTs observed in 2005, this is still a lot of extra heat energy. SSTs remain depressed over the southern Gulf of Mexico and western Caribbean, due to the cold water wake of Hurricane Dean. Wind shear the past 11 days (Figure 3) has been below normal over most of the MDR. These conditions are expected to continue over at least the next two weeks, according to the latest forecast from the GFS model. African dust activity has been quite low the past month, and I don't see any changes to the general circulation pattern that would change this. Steering current patterns are expected to remain the same as we've seen since since late July, with a series of weak troughs and ridges rippling across the Atlantic, and no major troughs or ridges locking into place. This steering pattern favors a near-normal chance of hurricane strikes for the entire Atlantic. Due to the weak nature of the troughs of low pressure expected, we'll have fewer recurving storms that miss land than normal. Indeed, all but one of the seven named storms we've had this year have affected land (Chantal was the exception).

Figure 3. Departure of wind shear from average for the past 11 days (in meters per second). Wind shear is the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude) in meters per second (multiply by two to get the approximate wind shear in knots). Note that wind shear has been below average over most of the tropical Atlantic the past 11 days. Image credit: NOAA/CPC.

Bill Proenza returns to his old jobBill Proenza will return as head of the Southern Region of the National Weather Service, according to an article published in the a href=http://www.miamiherald.com/news/hurricane/sto ry/233190.html yesterday. Mr. Proenza was director of the National Hurricane Center between January and July this year, until he was forced out by a dispute over his claims about the rationale for replacing the aging QuikSCAT satellite and an unprecedented revolt by his staff. He will return to the job he had before the NHC directorship, managing nearly 1,000 National Weather Service employees in New Mexico, Texas, Oklahoma, Arkansas, Louisiana, Mississippi, Tennessee, Alabama, Georgia, Florida, the Commonwealth of Puerto Rico and the U.S. Virgin Islands. Proenza resumes his duties on September 23. Proenza's return to his old job was pushed by two powerful congressmen, Democratic Reps. Brad Miller of North Carolina and Nick Lampson of Texas.

Gabrielle gone; next system to watch is 91L

Tropical Storm Gabrielle limped ashore over North Carolina's Outer Banks Sunday afternoon as a minimal tropical storm, bringing wind gusts up to 61 mph. Strong upper-level winds ripped Gabrielle apart as it came ashore, and has reduced it to a tropical depression. Diamond Shoals buoy off the coast at Cape Hatteras recorded a few hours of minimal tropical storm force winds (35 knots, 40 mph), and 15 foot waves. The biggest impact from the storm was some isolated minor flooding due to rains around eight inches. Gabrielle's rains did not stretch far enough inland to have a significant effect on North Carolina's drought, unfortunately. Here is a list of the wind and rainfall totals for the storm:

Figure 1. Total precipitation from Gabrielle from the Morehead City radar. Rainfall amounts of up to 8.6 inches fell in isolated regions.

Tropical wave in the mid-Atlantic (91L)A strong tropical wave near 10N 38W, about 1000 miles west-southwest of the Cape Verdes Islands off the coast of Africa, is headed west to west-northwest at 10-15 mph. This system (91L) has the potential to develop into a tropical depression later this week. This morning's QuikSCAT pass showed a broad, elongated circulation and top winds of 25 mph. Satellite loops show some disorganized clumps of heavy thunderstorm activity. The disturbance is under about 15 knots of wind shear. Shear is forecast to remain near 15 knots over three days, which may allow some slow development. Later in the week, shear is expected to drop below 10 knots, and this could lead to a better chance of development. Both the HWRF and GFDL models predict that this will be a hurricane five days from now. This seems over-aggressive, given the wave's current state of disorganization, and the shear forecast. I think the earliest this would become a tropical depression is Wednesday.

A large extratropical storm over the mid-Atlantic between Europe and the U.S. is expected to pull 91L on a more northwesterly track by mid-week. This would put 91L in a position to threaten the northern Lesser Antilles Islands seven days from now.

Figure 2.Today's lineup of systems to watch.

Gulf of Mexico disturbance (90L)An area of disturbed weather in the southern Gulf of Mexico (90L) remains disorganized. Wind shear is not a major impediment to development--upper level winds from the north are creating about 10-15 knots of wind shear over the region, and this shear is expected to remain below 15 knots through Wednesday. However, given the extremely disorganized appearance of this disturbance on satellite loops, any development should be slow to occur. This system is headed west-northwest at 10-15 mph towards Texas.

Elsewhere in the tropicsAn area of disturbed weather near the northern Lesser Antilles Islands (designated 92L by NHC yesterday) has become disorganized and is no longer a threat. A new tropical wave with a closed circulation moved off the coast of Africa yesterday, but currently does not have much thunderstorm activity associated with it.

Gabrielle coming ashore in North Carolina

Tropical Storm Gabrielle is strengthening slightly as it approaches North Carolina's Outer Banks this morning. Radar animations from the Morehead City radar and satellite loops both show an increase in spiral banding, and an increasing amount of heavy thunderstorm activity on the southwest side of the storm. Wind shear of 15 knots caused by strong upper-level winds from the north are slowing the intensification process, and keeping the spiral bands from wrapping around to the north side of Gabrielle's center of circulation. Due to the shear and limited amount of time left before landfall, Gabrielle should come ashore with top winds no more than 50-55 mph. The land station that should receive the highest winds from Gabrielle will probably be Cape Lookout, which had sustained winds of 23 mph with a gust to 28 mph at 9am EDT. The Diamond Shoals buoy just off of Cape Hatteras had sustained winds of 34 mph and 11-foot high seas this morning. The greatest threat from Gabrielle will probably come from tornadoes it may spawn.

Figure 1. Total precipitation from Gabrielle from the Morehead City radar. Rainfall amounts of up to five inches will affect coastal North Carolina.

Gulf of Mexico disturbance (90L)An area of disturbed weather developed in the southern Gulf of Mexico last night, just north of the Yucatan Peninsula. This region was designated "90L" by NHC. Strong upper level winds from the north are creating about 15-20 knots of wind shear over the region, and this shear is expected to remain a rather high 15-25 knots through Monday afternoon. By Monday night, the shear is expected to drop below 10 knots, which may allow some development of 90L. The system is headed west-northwest towards Texas, and residents of Texas, northern Mexico, and Louisiana should monitor this system. None of the reliable hurricane genesis models develop 90L into a tropical depression.

Tropical wave off the coast of Africa (91L)A strong tropical wave near 10N 34W, a few hundred miles southwest of the Cape Verdes Islands off the coast of Africa, is kicking up some disorganized heavy thunderstorm activity. This disturbance was designated "91L" by NHC this morning. Last night's QuikSCAT pass showed a broad, ill-defined circulation associated with the disturbance. The disturbance is under about 10 knots of wind shear. Shear is forecast to remain below 10 knots over the disturbance over the next few days, and this system has the potential to develop as it heads west at 15 mph. Several of the computer models do develop this system, and predict that it may begin to acquire a more northwesterly track in response to a big non-tropical storm over the mid-Atlantic between Europe and the U.S. This disturbance has an environment similar to the one that spawned Hurricane Dean. The GFS-based SHIPS intensity model is forecasting that this will be a Category 1 hurricane 4-5 days from now, and I am expecting at least a tropical storm will form by then.

Gabrielle headed for the Outer Banks of North Carolina

Subtropical Storm Gabrielle formed last night, after a day-long struggle trying to figure out which of two centers to consolidate around. Gabrielle finally decided it would use both centers, and a very large and elongated circulation resulted. This makes Gabrielle a subtropical storm, since the heavy thunderstorm activity is well removed from the center of circulation. Satellite loops show that a long band of heavy thunderstorms to the north of Gabrielle is beginning to wrap into the center of the storm, meaning that Gabrielle is making the transition to a tropical storm. Once heavy thunderstorm activity becomes concentrated around the center, the storm will be able intensify. This intensification will be aided early Sunday morning when Gabrielle passes over the warmest waters of the Gulf Stream, about 100 miles off the coast of North Carolina. Wind shear is about 10 knots over Gabrielle, which should also aid in intensification. My estimate is that intensification will begin late this afternoon. Gabrielle will probably have time to intensify to a 55-60 mph tropical storm before landfall (or a close miss) along the Outer Banks of North Carolina.

This morning's QuikSCAT pass mostly missed Gabrielle, but did note winds up to 30 knots (35 mph) along the storm's west side. The Hurricane Hunters are not due back in Gabrielle until 2pm EDT this afternoon. The NOAA jet is scheduled to fly tonight. The storm's outer rain band is now visible on Morehead City long-range radar.

Recent model runs have been pushing the track of Gabrielle further and further out to sea, and there is a good chance the storm will miss making a direct hit on the U.S. Regardless, most of the North Carolina coast from Wilmington to Kill Devil Hills will experience heavy rain of 1-4 inches. Tropical storm-force winds and a 3-4 foot storm surge will also affect some areas. Gabrielle is currently (Figure 1) producing rainfall amounts of about one inch every three hours along its northern band (Figure 1). These rainfall amounts should increase by about 50% by tomorrow as the storm grows more tropical in nature. Unfortunately, Gabrielle's rains will not penetrate very far inland to help alleviate the North Carolina drought. It now appears that Gabrielle will not affect New England, but could bring tropical storm force winds to Newfoundland early next week as an extratropical storm.

Figure 1. Estimates of rainfall over a 3-hour period ending at 8pm EDT last night. Note the higher rainfall rates present in the tropical disturbance just south of the Cape Verdes Islands. Tropical systems produce more rain than subtropical storms. Image credit: NASA.

I'll have a new blog later today if there's a significant change to Gabrielle. Otherwise, I'll post by Sunday morning at 10am. There is a tropical wave off the coast of Africa that may develop to talk about.

Gabrielle forms

Subtropical Storm Gabrielle formed tonight, after a day-long struggle trying to figure out which of two centers to consolidate around. Gabrielle finally decided it would use both centers, and a very large and elongated circulation is the result. This makes Gabrielle a subtropical storm, since the heavy thunderstorm activity is well removed from the center of circulation. Had the storm been able to consolidate around a single center, it would have become a tropical storm, not a subtropical storm.

Wind shear is 10-20 knots over the storm. This shear, combined with the rather large and poorly-organized circulation, will not allow much intensification, and Gabrielle will have a tough time becoming stronger than a 55-mph storm at landfall.

Carolinas disturbance struggling with two centers

The area of disturbed weather (99L) moving towards the North Carolina/South Carolina coast is trying to get more organized, but has developed a split personality. With a west and an east circulation center just a few hundred miles apart (Figure 1), the two areas of disturbed weather are competing, and neither appears able to gain the upper hand. 99L cannot become a tropical depression until one center becomes dominant. As a result, the prospects for 99L making landfall as a strong tropical storm are growing increasingly dim, and it now appears that a 55-mph tropical storm will be the strongest system that will have time to form. The timing of landfall is problematic, since it is not clear which center will end up winning. Wind shear is higher over the western center (20 knots) than the eastern center (10 knots), so this may favor the eastern center winning out. The western center's thunderstorms should begin affecting the South Carolina/North Carolina coast Saturday night, and the the eastern center will begin affecting this region Sunday morning. Top winds found by the latest Hurricane Hunter aircraft around 7pm EDT were approximately 30 mph at the surface.

Figure 1. Satellite image of 99L showing the east and west centers competing to be boss. Whose side will the blob of thunderstorms in the middle choose?

Polar ice cap gone by 2030?

July's huge drop in Arctic sea ice extent continued into August 2007, according to figures released this week by the National Snow and Ice Data Center. August 2007 sea ice extent plunged 31% compared to the average from 1979-2000. As of September 4, 2007, the sea ice extent was a full 17% below the record minimum that occurred on September 20-21, 2005. Although the rate of melting is starting to slow down as the days grow shorter, more melting is expected this month.

The difference in sea ice extent between August 1979 (the beginning of the data record) and August 2007 was a startling 37%. University of Illinois Polar Research Group presented similar estimates this week. They measure sea ice area--not extent. Sea ice area does not include all the long, narrow cracks in the ice, and so the numbers for sea ice area are different (lower) than for sea ice extent. Their sea ice area estimate for September 5, 2007 (Figure 1) was 42% less than for the same date 28 years ago.

An ice-free Arctic in just 23 years?None of our computer climate models predicted that such a huge loss in Arctic ice would occur so soon. Up until this year, the prevailing view among climate scientists was that an ice-free Arctic ocean would occur in the 2070-2100 time frame. The official word on climate change, the February 2007 report from the U.N.-sponsored Intergovernmental Panel on Climate Change (IPCC), warned that without drastic changes in greenhouse gas emissions, Arctic sea ice will "almost entirely" disappear by the end of the century. This projection is now being radically revised. Earlier this year, I blogged about a new study that predicted abrupt losses of Arctic sea ice were possible as early as 2015, and that we could see an ice-free Arctic Ocean as early as 2040. Well, the Arctic Ocean has suffered one of the abrupt losses this study warned about--eight years earlier than this most radical study suggested. It is highly probable that a complete loss of summer Arctic sea ice will occur far earlier than any scientist or computer model predicted. In an interview published yesterday in The Guardian Dr. Mark Serreze, and Arctic ice expert with the National Snow and Ice Data Center, said: "If you asked me a couple of years ago when the Arctic could lose all of its ice, then I would have said 2100, or 2070 maybe. But now I think that 2030 is a reasonable estimate. It seems that the Arctic is going to be a very different place within our lifetimes, and certainly within our children's lifetimes." While natural fluctuations in wind and ocean circulation are partly to blame for this loss of sea ice, human-caused global warming is primarily to blame. In the words of Dr. Serreze: "The rules are starting to change and what's changing the rules is the input of greenhouse gases. This year puts the exclamation mark on a series of record lows that tell us something is happening."

The implicationsThe melting of the Arctic sea ice will not raise ocean levels appreciably, since the ice is made up of frozen sea water that is floating in the ocean. Sea ice melt does contribute slightly to sea level rise, since the fresh melt water is less dense than the salty ocean water it displaces. According to Robert Grumbine's sea level FAQ, if all the world's sea ice melted, it would contribute to about 4 millimeters of global sea level rise. This is a tiny figure compared to the 20 feet of sea level rise locked up in the ice of the Greenland ice sheet, which is on land.

The biggest concern about Arctic sea ice loss is the warmer average temperatures it will bring to the Arctic in coming years. Instead of white, reflective ice, we will now have dark, sunlight-absorbing water at the pole, leading to a large increase in average temperature. Warmer temperatures will accelerate the melting of the Greenland ice sheet, which holds enough water to raise sea level 20 feet. The official word on climate, the 2007 IPCC report, predicted only a 0.6-1.9 foot sea level rise by 2100, due to melting of the Greenland ice sheet and other factors. I believe these estimates will need to be revised sharply upwards in light of the unexpectedly high Arctic sea ice loss this summer.

One more point--global warming skeptics often criticize using computer model climate predictions as a basis for policy decisions. These models are too uncertain, they say. Well, the uncertainty goes both way--sometimes the models will underestimate climate change. We should have learned this lesson when the ozone hole opened up--another case where the models failed to predict a major climate change. The atmosphere is not the well-behaved, predictable entity the models try to approximate it as. The atmosphere is wild, chaotic, incredibly complex, and prone to sudden unexpected shifts. By pumping large amounts of greenhouse gases into the air, we have destabilized the climate and pushed the atmosphere into a new state it has never been in before. We can expect many more surprises that the models will not predict. Some of these may be pleasant surprises, but I am expecting mostly nasty surprises.

Felix's aftermath; 99L less organized

Hurricane Felix was not the catastrophe feared when it slammed ashore in Nicaragua as a Category 5 hurricane two days ago. While rains have been heavy over Nicaragua, Honduras, and surrounding countries, they have not been heavy enough to trigger widespread flooding and mudslides like Hurricane Mitch of 1998 and Hurricane Fifi of 1974 did. Rainfall estimates for the 24 hours ending at 8am EDT yesterday (Figure 1) show that rains of up to 125 mm (5 inches) fell in only a few isolated areas, and additional rains of 125 mm (5 inches) were the most expected from Felix.

While this is great news, particularly for Honduras, Nicaragua has suffered a severe blow in the Puerto Cabezas region where the eye of Felix came ashore. Nearly every structure in Puerto Cabezas sustained at least roof damage, and many buildings were destroyed. Along the Miskito Coast of northeast Nicaragua, flooding and mudslides were reported, destroying many homes and blocking highways. The Government of Nicaragua declared the northern Caribbean coast a disaster area. At least 48 people have been reported dead--47 of them in Nicaragua, and one in Honduras (in a motor vehicle accident caused by heavy rain and landslides). However, dozens are missing (mostly at sea), and communications are difficult to impossible in many areas. At least 40,000 people have been affected and 9,000 houses destroyed, most of them in the Nicaraguan city of Puerto Cabezas, where a "State of Disaster" has been declared by the government.

Figure 1. Satellite estimated rain amounts for the 24 hours ending at 8am EDT Wednesday September 5 (12 GMT). Maximum rain amounts of about 125 mm ( 5 inches) were estimated over a few spots due to Felix. Image credit: Navy Research Lab.

U.S. East coast watches tropical disturbance 99LAn area of disturbed weather (99L) that formed along an old frontal boundary has grown less organized over the past 24 hours, thanks to an increase in wind shear. Strong upper level winds from the west are creating about 20-25 knots of wind shear over 99L, and satellite loops of 99L show a much less organized circulation, with only a few thunderstorms far removed from the center. The disturbance is interacting with an upper-level trough of low pressure, and this trough is creating a long line of thunderstorms from southwest to northeast that passes just east of the center of 99L's circulation.

Wind shear is forecast to drop below 15 knots on Friday, which may allow some slow development. I wouldn't be surprised to see 99L become a tropical storm on Saturday. Most of the computer models bring 99L to the coast of North Carolina on Sunday. This does not give it much time to develop, and it is unlikely 99L would be able to intensify into a hurricane. The storm may not develop into a tropical cyclone at all, but even as a non-tropical storm, residents of the Carolinas can expect heavy rain and high winds on Sunday from this system. The storm is then expected to track northward and then northeastward along the coast, bringing heavy rains to the mid-Atlantic and New England areas on Monday and Tuesday. The Hurricane Hunters are scheduled to investigate 99L at 2pm EDT today, but NHC may cancel this flight unless 99L shows some significant improvement in organization.

African development?The models are less gung-ho today about developing a tropical depression near the Cape Verdes Islands off the coast of Africa 2-4 days from now. As always, this region will need to be watched.

Riders of the stormNOAA P-3 Orion Hurricane Hunter airplane N42RF (AKA "The Princess") had a wild ride in Hurricane Felix on September 2 as it intensified into a Category 5 storm. The airplane hit what the Hurricane Hunters fear most--a powerful updraft followed a few seconds later by an equally powerful downdraft. The resulting extreme turbulence and wind shear likely made the aircraft impossible to control. Four G's of acceleration in both the up and down directions battered the airplane, pushing it close to its design limit of 6 G's. Although no one was injured and no obvious damage to the airplane occurred, the aircraft commander wisely aborted the mission and N42RF returned safely to St. Croix. The airplane was out of commission for the next day, which unfortunately meant that wunderblogger Mike Theiss was unable to get on his scheduled flight into Felix (he's written a blog today about this disappointment). However, the aircraft has now passed a detailed six-hour inspection to look for turbulence damage, and has been cleared to fly again. Welcome back, Princess!

Figure 2. Still shot from the 7-minute video of the Air Force Hurricane Hunters' penetration of Hurricane Felix at 06 GMT (2am EDT) September 3, 2007. Felix had just intensified into a Category 5 hurricane with 165 mph winds. Lightning in the eye is lighting up the eyewall clouds here.

Wunderblogger Randy Bynon, a meteorologist with the Air Force Hurricane Hunters, flew through Hurricane Felix on the mission that followed N42RF's. He and Lt Colonel Scott Dufreche have posted a 7-minute video of their flight through Felix. The camera looks forward through the cockpit windows, and one can see the cockpit instruments and the view out the front window. The video starts about 20 miles outside the eye, and during the 4-minute penetration of the eyewall, steadily increasing turbulence shakes the C-130 aircraft and frequent flashes of lightning light up the eyewall clouds. After four minutes, the turbulence suddenly slackens as the airplane breaks into the eye, and you can see spectacular glimpses of the eyewall clouds lit up by lightning flashes. The video finishes with some radar screen shots and fish-eye views of the top of the eye showing the full moon. Amazing stuff!

99L a threat to Carolinas; Felix dies; Henriette does a double landfall

Hurricane Felix is no more. The high mountains of Honduras have dissipated the storm, just 24 hours after Felix smashed ashore near the Nicaragua/Honduras border with 160 mph winds and an 18-foot storm surge. Puerto Cabezas, a Nicaraguan Caribbean coast town of 40,000 people, took the worst of Felix's wrath. The town sits just 10 miles south of where the eye hit, and preliminary reports indicate much of the town was heavily damaged, and three people were killed. The big fear from Felix continues to be the heavy rains it is spawning over the mountainous regions of Nicaragua, Honduras, and El Salvador. So far, satellite estimates of rainfall show amount up to 125 mm (5 inches) have fallen, with up to another 5 inches expected by 8pm tonight (Figure 1). It is unlikely that rains of this magnitude can trigger the type of catastrophes suffered by the region during Hurricane Mitch and Hurricane Fifi. I am hopeful that Felix's major destruction will be confined to the small region near landfall in northeastern Nicaragua.

Figure 1. Forecast rain amounts for the 24 hours ending at 8pm EDT Wednesday September 5 (00 GMT September 6). Maximum rain amounts of about 125 mm ( 5 inches) are predicted over the region where Honduras, Nicaragua, and El Salvador meet, and also near the Belize/Guatemala border.

Carolinas at risk from tropical disturbance 99LAn area of disturbed weather (99L) that formed along an old frontal boundary appears to have developed into a subtropical depression, and may grow into a tropical storm over the next day or two. Strong upper level winds from the west are creating about 15 knots of wind shear over 99L, and satellite loops of 99L show the classic appearance of a weak, sheared system--a nearly exposed low level circulation system, with all the heavy thunderstorm activity pushed to one side by strong upper-level winds. This shear is forecast to remain between 15 and 25 knots over the next two days, which should allow some slow development. A QuikSCAT pass from 6:52am EDT showed that 99L has a vigorous closed circulation with top winds of 25-30 knots (30-35 mph), so in my book this system is already a subtropical depression. The reason I call it subtropical is because there is still clear evidence of a frontal boundary attached to 99L, evident as long band of clouds extending from the south side of the storm (Figure 2). The Hurricane Hunters are scheduled to investigate this system at 2pm EDT, and NHC may wait until then to see if 99L can maintain its strength before naming it a subtropical depression.

The computer models are all unanimous in developing 99L into a tropical storm. The preferred tracks are into North Carolina or South Carolina by Sunday or Monday. The HWRF, SHIPS, and GFDL intensity models are calling for a weak tropical storm, strong tropical storm, and Category 2 hurricane, respectively, when 99L makes landfall Monday in the Carolinas. Residents of the east coast of the U.S., and the Carolinas in particular, should carefully watch the development of 99L.

African development?The models unanimously forecast a tropical depression will develop near the Cape Verdes Islands off the coast of Africa 2-4 days from now. The models showed a similar degree of unanimity for the development of Hurricane Dean in a similar situation, so the chances of another named storm off the coast of Africa early next week are considerable.

Henriette hits Baja, heads for mainland MexicoHurricane Henriette hit the southern tip of Mexico's Baja as a Category 1 hurricane yesterday, and is on its way to a second landfall in mainland Mexico later today. This would make Henriette Mexico's second double landfalling hurricane this season--Hurricane Dean made a double landfall along the Atlantic side of Mexico, hitting the Yucatan Peninsula and mainland Mexico in the Bay of Campeche. Guasave radar and satellite loops show that Henriette is not very well-organized, but the hurricane should be able to maintain its strength over the warm waters of the Sea of Cortez and come ashore as a weak Category 1 hurricane or strong tropical storm. Henriette killed one person due to high surf in Cabo San Lucas yesterday, and six people in landslides in Acapulco Saturday. The remains of Henriette could bring heavy rains to Arizona, New Mexico, and western Texas late this week.

Category 5 Felix slams Nicaragua and Honduras

A strengthening Hurricane Felix powered ashore as a Category 5 hurricane near the Nicaragua/Honduras border at 8am EDT this morning, bringing 160 mph winds and an 18-foot storm surge to this sparsely populated region. Felix weakened for a period yesterday as it became too tightly would to maintain its eyewall, but a new eyewall formed last night in time for Felix to regain Category 5 strength before landfall. This year marks the first time in recorded history that two Category 5 storms (Felix and Dean) have made landfall in the Atlantic basin in the same year. Since reliable record keeping began in 1944, there have been 27 Category 5 hurricanes in the Atlantic. Eight of these have occurred in the past five years.

Figure 1. Moonrise over the eyewall of Hurricane Felix as it intensified into a Category 5 hurricane Sunday night. Wunderblogger Randy Bynon has more great photos in his blog where he recounts his mission into Hurricane Felix. He was scheduled to fly again today, so he may have more photos tonight of Felix inside the eye as it made landfall in Nicaragua.

Central America will be hard-hit by FelixFelix's rains will cause serious flooding and dangerous mudslides in Central America all along its path. But, it could have been much worse. Felix came ashore in a sparsely populated area, and damage from the storm's extreme winds and storm surge will mostly affect empty marshlands. As seen in the wind analysis of Felix at landfall (Figure 2), hurricane force winds are confined to a very small region near the center, and it is doubtful that any major cities will experience even Category 1 hurricane winds. Puerto Cabezas, Nicaragua, just 10 miles south of where the eye hit, is still reporting, and has thus far reported sustained winds of 50 mph.

The big worry is the rains from Felix. So far, satellite loops are showing that Felix has not yet tapped into the Pacific Ocean as a source of moisture. Both Hurricane Fifi of 1974 and Hurricane Mitch of 1998 were large and slow moving enough that they were able to draw in large quantities of warm, moist air from the Pacific over the mountains of Honduras. Felix just has Atlantic Ocean moisture to feed its rains, and this should prevent the storm from causing the kind of flooding catastrophe Fifi and Mitch did. With the eye of Felix over land and the storm weakening, I don't believe the storm will be able to match the 20+ inch rainfall totals of those storms. Even so, the 5-10 inches of rain predicted by NHC will cause severe flooding and dangerous landslides throughout Honduras and portions of Nicaragua, Guatemala, Belize, El Salvador, and Mexico. For the next 24 hours, this flooding will be confined to eastern Honduras and northern Nicaragua (Figure 3).

East coast of the U.S. at risk from new tropical disturbanceAn area of disturbed weather formed off the north coast of Florida yesterday, and this disturbance has been designated 99L by NHC. Strong upper level winds from the west are creating about 20 knots of wind shear over 99L, and satellite loops show that these winds are keeping all of 99L's heavy thunderstorm activity pushed over to the southeast quadrant of the storm. This shear is forecast to remain between 15 and 25 knots over the next five days by the GFS model, so any development of 99L should be slow. Despite the relatively high shear, the computer models are mostly calling for 99L to develop. Steering currents are weak in the region, and the models agree that 99L is likely to make a clockwise loop over the next three days, then potentially threaten (take your pick):

UKMET: North Carolina on SaturdayNOGAPS: Florida on FridayHWRF: New York on SaturdayECMWF: South Carolina on FridayCanadian: North Carolina on Saturday

The Hurricane Hunters are scheduled to investigate this system at 4pm EDT Wednesday.

98LThe tropical wave in the mid-Atlantic, halfway between Africa and the Lesser Antilles (98L), couldn't hold together its circulation any longer in the face of four days of wind shear and dry air. The disturbance has degenerated into a loose swirl of disorganized clouds. There is still some rotation evident on satellite imagery, and this region will need to be watched over the next few days.

Henriette closes in on BajaHurricane Henriette finally took advantage of the favorable environment for intensification it has been in, and developed enough of an eye to be labeled a hurricane. Los Cabos radar and satellite loops show the eye clearly, and some additional organization appears to be going on. An Air Force Hurricane Hunter aircraft is scheduled to provide more information this afternoon, before Henriette comes ashore on the southern tip of the Baja Peninsula tonight. Henriette may have time to intensify to Category 2 status before making landfall in Baja, and again before making a second landfall along the Gulf of California coast in Mainland Mexico. Henriette killed one person due to high surf in Cabo San Lucas yesterday, and six people in landslides in Acapulco Saturday. The remains of Henriette could bring heavy rains to Arizona, New Mexico, and western Texas late this week.

Felix roughs up the Hurricane Hunters, takes aim at Central America

Hurricane Felix put on an incredibly ferocious burst of intensification last night, winding up into a small but potentially catastrophic Category 5 hurricane. Felix is the second-fastest storm in the Atlantic to intensify to Category 5 strength from a tropical depression. Felix required just 51 hours to reach Category 5 strength after it started as a tropical depression. That is a truly remarkable intensification rate, considering most tropical cyclones take 3-5 days to organize into a Category 1 hurricane. The only hurricane that intensified faster was Hurricane Ethel of 1960.

Figure 1. Satellite image of Felix as it entered its rapid intensification cycle to become a Category 5 hurricane. A NOAA P-3 Hurricane Hunter aircraft was forced to abort its mission because of extreme turbulence shortly after this photo was taken.

Hurricane Hunters walloped by FelixNOAA Hurricane Hunter aircraft N42RF experienced a truly awesome and terrifying mission into the heart of Hurricane Felix last night. Flying at 10,000 feet through Felix at 7pm EDT, N42RF dropped a sonde into the southeast eyewall. The swirling winds of the storm were so powerful that the sonde spun a full 3/4 circle around the eye before splashing into the northwest eyewall. It is VERY rare for a sonde to make nearly a complete circle around the eye like this. As the plane entered the eye of the now Category 5 hurricane, they found a 17-mile wide stadium lit up by intense lightning on all sides. The pressure at the bottom of the eye had hit 934 mb, and the temperature outside, a balmy 77 degrees at 10,000 feet. This is about 24 degrees warmer than the atmosphere normally is at that altitude, and a phenomenally warm eye for a hurricane. N42RF then punched into the northwest eyewall. Flight level winds hit 175 mph, and small hail lashed the airplane as lighting continued to flash. Then, the crew hit what Hurricane Hunters fear most--a powerful updraft followed a few seconds later by an equally powerful downdraft. The resulting extreme turbulence and wind shear likely made the aircraft impossible to control. Four G's of acceleration battered the airplane, pushing the aircraft close to its design limit of 6 G's. Although no one was injured and no obvious damage to the airplane occurred, the aircraft commander wisely aborted the mission and N42RF returned safely to St. Croix. N42RF is the same aircraft that survived a pounding of 5.6 g's in the eyewall of Hurricane Hugo in 1989. A special inspection of the aircraft is planned for today to determine if it is fit to fly further missions into Felix, and its scheduled afternoon flight into the hurricane was canceled. Hurricane Hunter missions since have fared better, and no more extreme turbulence has been reported.

Some good news for Central America Although Hurricane Felix has intensified into a truly awesome and potentially catastrophic hurricane, the prospects of a major flooding catastrophe in Honduras and Nicaragua are much lower today than they appeared yesterday. Firstly, the dramatic intensification cycle has spun Felix into a very small, tight coil. A storm of this small size is much less likely to pull in moisture from the Pacific Ocean over the mountains of Honduras like Hurricane Fifi of 1974 did. Secondly, the strong ridge of high pressure pushing Felix westward has intensified, resulting in a greater forward speed for the hurricane. Felix is now moving at 21 mph, and will not slow much during its passage over Honduras. This will keep rainfall amounts lower than I expected. Thirdly, it now appears likely that Felix will hit the Honduras/Nicaragua border area, a very sparsely populated region known as "The Mosquito Coast". The region is mostly a large expanse of marshy wetlands. This track will result in the rapid weakening of Felix, limiting the rainfall from the storm. A passage just north of Honduras would have been far more disastrous. It will still be bad for Honduras--NHC is predicting 5-8 inches of rain, with isolated amounts up to 12 inches--but this is far short of the 20+ inches of rain that fell during catastrophic Hurricane Fifi and again in 1998 during Category 5 Hurricane Mitch. The nations of Nicaragua, El Salvador, Guatemala, Belize, and Mexico can also breather easier, as Felix's rains should not cause the kind of extreme flooding a larger storm would have caused. It now appears the Felix will stay too far south to be influenced by the trough of low pressure forecast to move across the U.S. later this week, so Texas appears safe from the storm.

98LNot much has changed with the tropical wave (98L) in the mid-Atlantic, halfway between Africa and the Lesser Antilles since yesterday, so I will mostly repeat yesterday's discussion. The system has a closed circulation and a small area of heavy thunderstorm activity on the west side of the center. Wind shear of 15 knots from strong upper-level winds from the east-southeast are preventing thunderstorm activity from building on the east side of the storm. Several of the reliable models are forecasting that this shear will fall below 15 knots by Tuesday. There is some dry air to the northeast for the disturbance to contend with, but I expect 98L will be able to overcome this dry air and shear and organize into a tropical depression. Thursday is the earliest this would happen. The UKMET is the only model that develops 98L into a tropical depression.

98L is nearly stationary, and it will be at least six days before it will threaten the Lesser Antilles Islands. A strong trough of low pressure will pass north of 98L Tuesday and Wednesday, which could impart a more northwesterly motion to the storm.

Elsewhere in the tropical AtlanticSeveral computer models develop a tropical depression off the South Carolina coast by Wednesday or Thursday, along an old frontal boundary. An area of disturbed weather has already formed here, and will bear watching over the next few days. You can track this using long range radar out of Jacksonville, Florida. The eventual track such a storm might take is highly uncertain--the NOGAPS foresees a threat to North Carolina, the UKMET and ECMWF has the system looping back and hitting Florida, while the GFS has the storm heading out to sea near Bermuda.

Henriette takes aim at BajaTropical Storm Henriette continues to churn along the Pacific coast of Mexico towards Baja, and could bring hurricane conditions to Baja on Tuesday. Despite a favorable environment, Henriette has not been able to intensify into a hurricane. Visible satellite loops show increased thunderstorm activity wrapping around the center, so the Hurricane Hunters may find a hurricane when they arrive at the storm this afternoon. The remains of Henriette could bring heavy rains to Arizona on Friday.

Extremely dangerous Felix heads for Honduras; two other disturbances may develop

Hurricane Felix grazed the islands of Aruba, Bonaire, and Curacao this morning, passing 50 miles north of the northernmost island in the chain, Aruba. Thus far, top winds reported on Aruba have been 28 mph, gusting to 40 mph. Bonaire's winds peaked at 22 mph, and top winds at Curacao were only 18 mph. Heavy rains were reported on all three islands, but damage should be minimal. Felix missed--but not by much (Figure 1).

Felix a possible catastrophe for HondurasGiven the nearly ideal conditions for intensification--low shear and plenty of high-heat content water--Felix is likely to be a Category 4 storm as it approaches Honduras Monday. If the center tracks parallel to the coast within about 100 miles of it, an extremely dangerous situation ensues for Honduras. Hurricane Fifi of 1974 passed along the north coast of Honduras in 1974 as a Category 2 hurricane, and dumped up to 24 inches of rain on the mountainous country. The resulting landslides and floods killed an estimated 8,000 people--the fourth deadliest Atlantic hurricane ever. The town of Choloma in northwestern Honduras suffered the most. A huge mudslide triggered by Fifi's rains plowed into the city at dusk on September 20, 1974. The mudslide then formed a dam that pent Fifi's raging flood waters. When the dam burst, flood waters ravaged the entire city. Half of Choloma's population--about 2,800 people--died in the catastrophe. Fifi also killed at least 200 people in neighboring Guatemala.

Felix will be stronger that Fifi was, but it will be the size of Felix that will be critical in determining if Fifi-like rains hit Honduras. As Fifi approached Honduras between September 17 and 18, 1974 (Figure 2), it grew in size. The increased size allowed Fifi to pull in moisture from the Pacific Ocean, which greatly enhanced the rainfall over Honduras. If Felix grows large enough to tap this Pacific source of moisture, prodigious rains capable of causing major loss of life will result. Currently, Felix is not large enough to tap the Pacific moisture source. Additionally, Felix is moving faster--17 mph, as opposed to Fifi's 11 mph. This won't give Felix as much time to pour torrential rains on the region. These factors may combine to prevent Felix from matching Fifi's deadly rainfall totals. However, once Felix moves clear of the South American coast, there is a good chance the storm will grow in size, due to the additional influx of moisture from the south. If this occurs, not only Honduras is at risk. The nations of Nicaragua, El Salvador, Guatemala, Belize, and Mexico will also be at high risk of life-threatening flash flooding and mudslides.

Figure 2. Satellite photos of Hurricane Fifi of 1974, which killed 8000 people in Central America. Fifi was the fourth deadliest hurricane in Atlantic history. Note how in the right hand photo, taken 24 hours after the left hand photo, Fifi had tapped the Pacific Ocean as a source of moisture, and the storm's spiral bands reach all the way to the Pacific.

Felix's threat to other localesFelix will be devastating wherever it makes landfall, which currently appears to be the Yucatan Peninsula. There are no troughs of low pressure capable of altering Felix's steady course coming, until the storm emerges into the Gulf of Mexico after crossing the Yucatan. At that point, a trough of low pressure strong enough to bring Felix to the Texas coast may move through. It is too early to guess how strong this trough might be, and what the potential risk to Texas is.

98LA tropical wave (98L) in the mid-Atlantic, halfway between Africa and the Lesser Antilles, has the potential to develop into a tropical depression later this week. However, conditions are not as favorable today as they appeared yesterday, and there is a higher likelihood that wind shear and dry air will prevent this disturbance from developing. The system has a closed circulation and a small area of heavy thunderstorm activity on the west side of the center. Wind shear from strong upper-level winds from the east-southeast are preventing thunderstorm activity from building on the east side of the storm. The SHIPS model was forecasting a low shear environment would set up over the path of 98L by Tuesday, but now has reversed itself, calling for shear to remain 15-25 knots for the next five days. If the shear does remain that high, formation of a tropical depression is unlikely. However, 98L has a vigorous circulation, and it may be able to outlast the hostile wind shear until it finds a more favorable environment. Furthermore, other global models, such as the NOGAPS, are not forecasting such hostile wind shear conditions. I believe the SHIPS model shear forecast looks a little fishy, and that the NOGAPS model has the right idea. I'm expecting 98L to be a tropical depression later this week, perhaps as early as Wednesday.

98L moved southwestward for a period last night, but now has resumed its westward motion and slowed down to 10 mph. A strong trough of low pressure will pass north of 98L Tuesday and Wednesday, which could impart a more northwesterly motion to the storm. The GFDL and HWRF models have 98L moving to a position about 800 miles east of Puerto Rico by Thursday. A ridge of high pressure should build in after the trough passes, forcing 98L on a more westward track towards Puerto Rico. This is highly speculative, as both the HWRF and GFDL develop 98L into a 50 mph tropical storm by mid-week, and this is not likely to occur. I expect a weaker 98L may not "feel" the presence of the trough so strongly, and it may not get pulled as far north as the GFDL and HWRF are forecasting. In this case, 98L would stay farther south and move through the Lesser Antilles south of Puerto Rico late this week. None of the other computer models develop 98L.

Elsewhere in the tropical AtlanticSeveral computer models develop a tropical depression off the South Carolina coast by Wednesday or Thursday, along an old frontal boundary. An area of disturbed weather has already formed here, and will bear watching over the next few days. You can track this using long range radar out of Jacksonville, Florida. The Cape Verde Islands off the coast of Africa are also predicted to be a genesis region, as early as Tuesday.

Henriette kills six in Acapulco, takes aim at BajaTropical Storm Henriette continues to churn along the Pacific coast of Mexico towards Baja this morning, and could bring hurricane conditions to the Baja on Tuesday. Now that Henriette has moved away from the coast, it should be able to organize itself and intensify, possibly into a major Category 3 hurricane by Tuesday. Visible satellite loops show this intensification may be underway. Heavy rains from Henriette triggered two landslides in Acapulco Saturday that killed six residents.

Felix a major threat to Central America; new disturbance could develop

Tropical Storm Felix lashed the islands of Grenada, Trinidad, Tobago, Barbados, and St. Vincent and the Grenadines with winds near tropical storm force and torrential rains this morning. Visible satellite loops show that Felix is a small storm, but is steadily expanding in size and growing more organized. Low level spiral bands have formed on the eastern side, and there is one respectable upper-level outflow jet that has formed to the storm's north. Dry air on the northwest side of Felix continues to hamper its intensification, but the storm is small enough that dry air drawn in from the north coast of South America has not been a problem.

Felix is a major danger to Central AmericaThe latest GFDL model forecasts that Felix will intensify into a Category 2 hurricane by the time it makes landfall in Belize Wednesday. The SHIPS intensity model is more aggressive, making Felix a Category 3 hurricane. Given that the environment in the Caribbean is much the same as we saw for Dean, I think we can expect a steady intensification of Felix to a Category 2 or 3 storm when it approaches the Honduras/Nicaragua border Monday night. On the current projected track of Felix, it would pass just north of the coast of Honduras, which would be an extremely dangerous situation for that country. Hurricane Fifi of 1974 passed along the north coast of Honduras in 1974 as a Category 2 hurricane (Figure 1), and dumped up to 24 inches of rain on the mountainous country. The resulting landslides and floods killed an estimated 8,000 people--the fourth deadliest hurricane disaster in the Atlantic basin. There is one important difference between Fifi and Felix--Fifi was moving slower, about 11 mph, Felix is expected to move past Honduras at about 17 mph, so will not linger as long to dump heavy rains. Even so, Felix's rains could reach 10-15 inches over Honduras, Nicaragua, Guatemala, and Belize. Officials in those nations need to prepare now for the possibility that Felix could bring a major flooding disaster to their nations.

Figure 1. Track of Hurricane Fifi of 1974, which killed 8000 people in Central America. Fifi was the fourth deadliest hurricane in Atlantic history.

Felix's threat to other localesFelix should being winds of tropical storm force to Aruba, Bonaire, and Curacao as it passes to the north. These islands, and the northern coast of Venezuela, will also get heavy rains, but Felix should not cause any serious wind damage or floods in those areas. The ridge of high pressure that is steering Felix to the west is strong enough that a northward deviation of the storm into Jamaica and the Cayman Islands is unlikely. If Felix is going to deviate from the projected NHC forecast the next two days, I think a southward deviation into Nicaragua is more likely.

If Felix does stay far enough north to make it into the Western Caribbean on Tuesday and Wednesday, there is a trough of low pressure forecast to swing north of the region that could turn Felix on a more northwesterly track into the Gulf of Mexico. The models are split on this, and we'll have to wait and see. Those of you planning on being in Cancun or Cozumel on Wednesday should pay close attention to Felix.

98LA tropical wave in the mid-Atlantic, halfway between Africa and the Lesser Antilles, has developed a closed circulation and some heavy thunderstorm activity near the center. This disturbance has been labeled "98L" by NHC this morning. The disturbance is under about 20 knots of winds shear from strong upper-level winds from the east-southeast, but this shear is forecast to gradually slacken over the next few days, and should be below 10 knots by Monday night, and under 5 knots by Wednesday. 98L is a threat to develop into a tropical depression as early as Monday. The 12Z (8am EDT) runs of the GFDL and HWRF models both develop 98L into a tropical storm, but keep it below hurricane strength. The storm will be approaching the Lesser Antilles Islands on Wednesday or Thursday. Given 98L's more northerly starting location, it may eventually affect Puerto Rico.